How old is the word "prolly"?

Solution 1:

Writer Hugh Wiley seems to have been using it in dialogue in the 1920's. From his 1924 book The Prowler (via Google Books):

Dese folks is prolly dem bank boys whut buys all de fruit farms and makes dem yaller orange preserve.

He uses it in his 1922 book Lily (via Google Books) as well:

"Looks like high tone folks. Visitin' prolly."

"Prolly is. Us goes down dis alley an' in de back way, Demmy.""

The characters using prolly in these books are supposedly speaking in the African-American Vernacular. Depending on how good an ear Wiley had for dialect, this may indicate prolly first arose in Southern or African-American speech.

Solution 2:

The three oldest occurrences in the Corpus of Historical American English are from 1947, 1952 and 1954 (see below). This therefore supports what the article claims. However, as @Peter Shor's answer shows, the word is actually even older (from 1922 at least).

(...) up with the rest of the cows at milkin time and he knowed she was prolly out there on the gully some'eres with a fresh-born calf, waitin for it to (...)

Harpers Magazine (194712) pages: 498-505 (1947)

(...) They say he wouldn't speak to Ma for a month after. He'd prolly have disinheritedme, seeing these, or shot Mott for a Texas traitor.(...) "

Giant, Edna Ferber (1952)

I'm quite aware that you come from a higher level of society. You prolly made a debut and all that.

Play BadSeed, Maxwell Anderson (1954)

Note that as Google NGrams shows, the word gained popularity in the Internet age:

Solution 3:

you might be looking for this 1947 citation?