What is the figurative meaning of "cold pizza"?

I've always thought that "cold pizza" was an expression used mainly to refer to a boring, uninteresting person or an unexciting event whose images call to mind that of an unappetising cold pizza.

According to Ngram "cold pizza" has been used increasingly from the early 70's.

The is no clear reference on this expression (just UD mentions it) and its figurative usage may lend itself to different interpretations at times, but I've noticed that it is often used referring to breakfast:

From The Amazing Adventures of a Marginally Successful Musician

• If you haven't had cold pizza for breakfast, you're not a musician and therefore, I'll refund your money for this book.

From The Crossroads: A Haunted Mystery:

• You know, in law school, we used to eat cold pizza for breakfast...

It is also the name of a TV sports morning talk show in the U.S.

Questions:

1) Is the expression actually used to refer to a boring, uninteresting person?

2) What are the other (if there are) figurative usages of "cold pizza"?

3) Is it mainly, as it appears, an AmE expression?

EDIT: I know that the expression comes from some eating habits of American people, my question is about the figurative usage of what a piece of cold pizza may suggest.

I undertook arduous, minute-long searches in the Ngram viewer (nothing) and on the google for search strings

"that's a cold pizza"
"it's a cold pizza"
"he's a cold pizza"
"was a cold pizza"
"is a cold pizza"

I got lots of restaurant reviews (mostly negative, of course), a link to the lyrics of the song "Cold Pizza Warm Beer" by Gaye Adegbalola

It’s just the morning after, I don’t remember who was here.
I’ll eat a slice of that cold pizza, and wash it down with warm beer.

references to the sports talk show on ESPN2, and instructions for making a palatable breakfast. Possibly the song lyric is metaphorical. I was about to give up when I ran into this from an article in Time Magazine (August 29, 2011) entitled "Another Slice of 'Cold Pizza'? The Man Most Likely to Lead Japan" about then Japanese Finance Minister Yoshihiko Noda, who was expected to become Prime Minister:

Can Noda lead? Analysts are not sanguine. "He emerged as a compromise," says Jeff Kingston, Director of Asian studies at Temple University Japan. "He's not charismatic, or a populist or a good communicator, [and] not a particularly bold or visionary leader. He's sort of a 'Steady Eddy' and doesn't raise expectations that much. ... Says Yoshi Yamamoto, a political advisor to DPJ [Democratic Party of Japan] congressmen: "Noda seems a bit like the former LDP [Liberal Democratic Party] prime minister Keizo Obuchi [in the 1990s], generally known internationally as a 'cold pizza,' but respected domestically by working level staffers and officials as someone who listens and rewards."

I can find no uses for Urban Dictionary's claims for sexual and drug connotations.

The majority of these usages are literal. If you had pizza, leftover from last night, you eat it cold in the morning for breakfast. It's not so much a metaphor as it is symbolic/representative of a unhealthy, undomesticated lifestyle. It means you're not eating a "real" breakfast, and you aren't even bothering to reheat the pizza. You're probably a hungover bachelor (or bachelorette).

An Elephind newspaper database search for "cold pizza" yields 196 matches—193 from the United States and 3 from Australia. The vast majority of the results are from college newspapers, which makes sense because most mainstream, general-circulation daily newspapers later than the early 1930s are excluded from the search results (for copyright reasons). The decade-by-decade matches for "cold pizza" work out as follows:

1950–1959: 5 matches

1960–1969: 10 matches

1970–1979: 25 matches

1980–1985: 45 matches

1990–1999: 28 matches

2000–2009: 76 matches

2010–2019: 7 matches

Notably, not one of these matches involves the use of "cold pizza" in a figurative sense or as a description of a person or activity. In every case case, "cold pizza" refers to pizza that is cold—either because the initial fresh-from-the-oven heat has dissipated (a process that takes less than 20 minutes, owing to the thinness of the typical pizza) or because the pizza is leftover and has either been stored in a refrigerator or left on a plate for hours (usually overnight).

References to "cold pizza" have evolved in several ways. First, the earliest Elephind matches for "cold pizza" seem to involve the cooled-off-from-its-original-warmness sense of the term—and all appear in the context of criticism. The oldest match is from an advertisement for Sam's Pizza Kitchen in the [Champaign, Illinois] Daily Illini (November 9, 1956), which reads simply:

Cold Pizza is for the birds! All Pizza delivered hot!

The earliest instance of an article referring to "cold pizza" as a dining option consisting of one or more slices left over from a meal eaten hours or days earlier is from Erma Bombeck, "Call It Pillow Talk," in the San Bernardino [California] Sun (November 22, 1971):

Notes pinned to the pillow of a mother who has flu — by a well-meaning husband who has inherited the house and kids.

...

Dear Doris: Why in the name of all that is sane would you put soap chips in the flour cannister! If you have time, could you please come up with a likely spot for Chris' missing shoes? We've checked the clothes hamper, garage, back seat of car and wood box. Did you know the school has a ruling on bedroom slippers? There's some cold pizza for you in a napkin in the oven drawer. Late tonight, Driving 8 Girl Scouts to tour meat packing house!

The concept of "cold pizza" for breakfast debuts in Elephind's newspaper database results in Clare Brown, "Clare's Column: Stack Up a Hearty Breakfast," in the Clare [Michigan] Sentinel (September 18, 1974):

Nowhere does it say breakfast has to be toast and eggs or cereal and milk, although these are excellent breakfasts, if these things turn kids off, try a hamburger deluxe, a bowl of soup, or a slice of cold pizza and milk. They fit into the good breakfast criteria just as well as our sterotyped concepts of what makes a breakfast.

The earliest match for "cold pizza [for breakfast]" as a representative lifestyle dietary choice for single men, college students, indie band members, computer geeks, etc., appears in "Magnetism plus equals Starsky & Hutch," in The Australian Women's Weekly (September 15, 1976):

The two of them seem to be the toughest and fittest plain-clothes detective commandos yet to grace TV's Los Angeles police force.

Maybe their breakfast helps them. In a recent episode Starsky had a breakfast of cold pizza and root beer (a rich, dark-brown soft drink).

From these origins, "cold pizza" has emerged as a cultural marker in roughly the same vague yet tendentious way that "brie and chablis" did (briefly) in the 1980s. Here is a typical instance, from Cheri Monaghan, "Going Home: Those Visits with the Parents Who Remind You to Wake Up in the Morning and Give You Gifts of Dog Halt," in the [University Park, Pennsylvania] Daily Collegian (February 7, 1986):

Remember the joy of newfound freedom away from screaming siblings and prying parents; staying out all night, cutting classes for days on end, playing the stereo at decibel levels only dreamed of when still at home, eating cold pizza and Pepsi for breakfast and doing a dozen other things our parents were likely to drop dead if aware of? At last we were really living!

And from Kevin Padrez, "It's OK, He's Just a College Student," in the Stanford [California] Daily (June 15, 2007):

After graduation I don't expect my lifestyle to change very much. I'm still going to eat cold pizza for breakfast and buy my formal wear at Savers. The only difference is I will now be considered a dead-beat twenty-something, not a typical college student.

Conclusion

In the United States, at least, "cold pizza" has not emerged as a widely recognized figurative expression. Instead, its force is as a shorthand representation of a particular (usually college-centered) dietary lifestyle. That the term retains this status almost half a century after its "it's what's for breakfast" debut is a testament to the somewhat surprising continuity of post-high-school eating habits, as channeled by young adults and as attributed to them by older ones.

My girlfriend and I often use this as an alternative way of saying "annoyed," based on the Noid commercials from Dominoes. For example, she'll say "I'm annoyed," and I say "Well, don't make my pizza cold." Or she'll say "my pizza's cold," and that means she's annoyed. It's kind of a light-hearted, tongue-in-cheek joke.

We have a similar thing with the phrase "go for it." Since it kind of sounds like "gopher it," I'll sometimes say "groundhog it" instead.