How did 'phat' come to be used in music as slang?
Slang Dictionary coverage of 'phat' (and 'fat')
The earliest slang dictionary entry for phat that I've found is from Connie Eble, College Slang 101 (1989):
PHAT Having a shapely body, applied to females. Popularly supposed from Pretty Hips and Thighs.
Five years later, two African-American slang dictionaries published in the same year offer contrasting definitions. From Clarence Major, Juba to Jive: A Dictionary of African-American Slang (1994):
Pfat (1990s) superb; really wise and classy; "cool"; "hip." Y[outh] C[ulture] U[se].
And from Geneva Smitherman, Black Talk: Words and Phrases from the Hood to the Amen Corner (1994):
PHAT ... Probably derived from "em-PHAT-ically"; also a play on the word "fat"
A year after that, Robert Chapman & Barbara Ann Kipfer, Dictionary of American Slang, third edition (1995) has this:
fat ... 5 adj (also phat) teenagers by 1951 Attractive; up to date; =Cool, DOPE, RAD | Fat is recorded by 1932 a[s] meaning "hot," in US dialect, and this may underlie the teenage use: If they are real fat, real crazy, naturally they're real cool—Newsweek [1951]/ Timberland boots have, in the parlance of the street, become "dope" and "phat," i.e. cool, greatest—New York Times 6 adj (also phat) 1980s students Sexy; having a shapely body | Some think this, when spelled phat, is an acronym for pretty hips and thighs: The three boys thought that Carolyn looked fat as she walked down the street—Delcastle [Technical High School, Wilmington, Delaware] Dictionary of Slang [1989]
The instance cited by Chapman & Kipfer from a 1951 issue of Newsweek appears in a longer article on teen slang. Here is a bit more of the article for context:
When talking, Utah bobbysoxers are bashing ears; when kissing, they are lumping lips. They seek out flicks (movies) and drive a wedge, a set of wheels or a rocket. When cutting classes they sluff. Chili means good deal; Dad is any young man, and drag it is let's hurry. Discussing the degrees of coolness, one Utah boy reported: "If you like a guy or gal, they're cool. If they are real fat, real crazy, naturally they're real cool." Salt Lake's highest accolade: frampton.
The New York Times citation in the same entry is evidently from the early 1990s.
Tom Dalzell, Flappers 2 Rappers: American Youth Slang (1996) has a lengthy discussion of phat. In the course of this discussion, the most striking point that the author makes is that the word—spelled phat—appears in a discussion of "Negro argot" in a 1963 issue of Time magazine. Here is a snippet from that Time magazine discussion:
Since slavery days, Negroes have created an ever-changing argot of their own, full of ambiguities, tinged with humor and sorrow. As might be expected, the Negro argot abounds in terms for the races:
greys, whities, paddies. Whites.
Mr. Charlie. A white man.
Miss Ann. A white woman.
...
fox, flavor. Pretty girl.
ace boon coon. Girl friend or buddy.
short. Automobile.
ragtop. Convertible.
stallion. A man who is handsome or husky or prosperous; also, a buxom woman.
mellow, phat, stone, boss. General adjectives of approval; a stone fox is a very pretty girl; a boss short is a big car.
Dalzell in Flappers 2 Rappers also offers this interesting observation:
As sometimes seems to be the case with slang that begins its journey meaning "rich," fat at some point began to take on a more general meaning of good. For example, American Speech in 1955 defined fat (shortened from "fat and happy") as "in an excellent situation." In his 1968 Hy Lit Dictionary, the archetypal fast-talking Philadelphia AM disc jockey [Hy Lit] included fat as "great, cool, you dig it."
'Phat' in a musical context: a 'phat bass line'
With regard to the posted question's focus on music-related use of phat, as in "a phat bass line," I note that—even in the mid-2000s (specifically 2005 and 2006)—slang dictionaries tended not to call out such use as entailing a special meaning, distinct from the meaning of phat in more general contexts. From Jeremy Sideris & Brittany McWilliams, From Grill to Dome: A Dictionary of African American Slang Words and Phrases (2005):
Phat: A state of excellence, or of graciousness.
And from Randy Kearse, Street Talk: Do Official Guide to Hip-Hop and Urban Slanguage (2006):
{something that's} phat adj. (general sl.) old & new school 1. outstanding; unique; the best of its kind. 2. hip; trendy; stylish (var. {something that's} phat to death/like a muhfka**) ex: "Now that was phat." "I think that's gonna be mad phat."
It also bears observing that the expression "fat bass line appears to be considerably older in U.S. published English than "phat bass line."
I found two early matches for "phat bass line" in Elephind and Google Books searches. From a review of Don Jagwarr's Faded in the Columbia [University (New York City)] Daily Spectator (December 2, 1994):
Despite a slow start, the album warms up with "Who Do You Fear?" This track, containing samples from A Tribe Called Quest's "Bonita Applebaum" and Digital Underground's "Humpty Dance," addresses the issue of seizing power through the use of a gun. "My Law" hits with a phat bass line. "Skank Wit' U" is definitely the most appealing song. This track makes you feel as though you are actually in the dance hall.
And from a review of Brandy's "Best Friend," in S2S Magazine (1995) [snippet view]:
I got one for you. This is Brandy's second single and it will match or do better that her smash single, "I Wanna Be Down." If you want to move this is the one. Phat beats, phat bass line and a good message. Brandy delivers with sultry singing. It's the total package. Check it out.
The earliest match for "fat bass line," are somewhat earlier. From a review of Andy Summers & Robert Fripp's I Advanced Masked in High Fidelity (December 1982) [combined snippets]:
This is particularly true of the title track and Hardy Country, which sounds like the former played backwards with a fat bass line and some percussion thrown in. This stuff is meant to be hypnotic, enticing ambient music, but as often as not it's simply headache inducing. To be fair there are other pieces—the spooky, humming Under Bridges of Silence, the ethereal Girl on a Swing—that are truly mesmerizing in a quiet kind of way.
From Larry Flick, "Into the Light: Gay Pride & the Club Community," in Billboard (June 27, 1992):
If you fancy a bit of Philly soul flavoring in your house music, don't miss "What Do You Want From Me" by Bobby Griffen (Cohiba). Remixes by Yvonne Turner maintain the original tune's retro angle while injecting highly programmable breaks and a fat bass line. Griffen possesses a rich baritone, and utilizes it to great effect here. Doesn't completely fit trendy mold, but it's a delightful interlude that requires a spin.
From a review of Original Flavor's "Here We Go," in Billboard (July 25, 1992):
You've certainly heard these party-boy rhymes before. But in this context, they are phrased in a relentlessly infectious manner. Cut's strongest element is the way it wraps a fat bass line around a live hip-hop drum pattern.
From a review of Mouth Music's Mo-Di in the [Santa Rosa, California] Oak Leaf (February 25, 1993):
Milking the Cow is track four. It has a quick percussion streak that drives the song. A fat bass line jumps in and the story begins. A mysterious voice that just seems to be floating around you comes in and takes the song even higher. It almost hums while it tells a story.
From a review of Joachim Kühn, Daniel Humair & J.-F. Jenny-Clark's Carambolage in Jazz Journal International (1993):
The Kuhn trio hits a brisk, easy swinging groove on the title track Carambolage with the band roaring intermittently behind them. Versage has tran[s]lucent piano, a beautifully firm, fat bass line and clean, shimmering cymbals whisking it along. The CMP recording is as clear as a bell and adds to the listener's enjoyment of this adventurous music.
And from a letter to the editor of Vibe (September 1993):
Thank you. You have finally given me a way to check out the black scene without being embarrassed, feared as a racist, or dumb. The writing in your magazine not only appealed to the fat-beat, fat-bass line, crazy-fat-lyric lover in me, but the college graduate (Morgan State University) in me was totally inspired as well.
Conclusions
The origins of phat are obscure and not made clearer by folk etymologies such as the "pretty hips and thighs" acronym theory and Geneva Smitherman's conjecture that it arose from "em-PHAT-ically."
The word appears (with the right spelling and in the relevant sense) in a 1963 discussion of "Negro argot" in Time magazine, but it also appears (with the wrong spelling and in the relevant sense) in a 1951 Newsweek article on teen slang—specifically, in the context of Utah teen slang. Chapman & Kipfer's 1995 edition of Dictionary of American Slang speculates that the teen slang use of fat in 1951 might itself be related to "US dialect" use fat in 1932 in the sense of "hot."
It's a pity that Chapman & Kipfer didn't identify the source of that recorded use or the particular dialect in which it occurred. Nevertheless, it seems quite possible that fat in the sense of "hot" in some unspecified U.S. dialect in 1932 found later expression in fat in the sense of "admirable or hip" in 1951 teen talk and again in phat in "Negro argot" in the sense of "worthy of approval" in 1963.
It does not appear from any of the sources that I consulted that phat (or fat) in a specifically musical context arose through a unique process of historical development that would distinguish it from the development of the general sense of phat (or fat) in the sense of "excellent, admirable, tasty, etc."
According to etymonline it was taken into hip-hop culture from street-slang for fat in a good or sexy sense. The rich and full bass meaning probably followed pretty naturally from there.
hip-hop slang, "great, excellent," 1992, originating perhaps in the late 1980s and meaning at first "sexiness in a woman." The word itself is presumably a variant of fat (q.v.) in one of its slang senses, with the kind of off-beat spelling preferred in street slang (compare boyz). The spelling is attested as far back as 1678, as an erroneous form of fat (a classical over-correction; see ph). This spelling is said by some to be an acronym, and supposed originals are offered: "pretty hot and tasty," or "pretty hips and thighs" among them, all unconvincing. These, too, may have been innovations given as explanations to women who felt insulted by the word.
The Merriam-Webster entry for phat says quite a lot about it.
phat adjective
slang
highly attractive or gratifying : EXCELLENT
a phat beat moving through my body — Tara RobertsMore examples of phat in a sentence
That song has a phat beat
Fans agree that the rapper's latest CD is totally phatFirst Known Use of phat
1963, in the meaning defined aboveHistory and Etymology for phat
probably alteration of fat entry 1