Green’s Dictionary of Slang

(US, orig. theatre) an old fogey.

1914		H. Hershfield Abie the Agent 18 May [synd. cartoon strip] See [...] if you can find mine old hat. It’s a little pancake with two initials A.K.
1920		Variety 27 Aug. 7: [heading] Thorek Cuts out Goat Gland Optimism. Theatrical surgeon says, Goat and Monkey N.G. for A.K.
1931		G. & I. Gershwin ‘Of Thee I Sing’ 🎵 We're the A.K.s who give the O.K.s One, two, three, four, five, six, seven, eight, nine Supreme Court Judges!
1943		‘Some American Idioms from the Yiddish’ in AS XVIII:1 Feb. 45: The ‘A.’ in ‘A.K.’ stands for alte, meaning ‘old,’ and the ‘K.’ for Kacker, defined in Harkavy, p. 297 of the Yiddish-English section, as ‘unfit person.’ [...] Variety, I think, is credited with having introduced the term in print and with having concocted that delicious definition, ‘antediluvian knight,’ which took in so many of the unsuspecting. [...] Actually, the term may refer to any old-fogey or has-been, whether he is an actor, a husband, or a superannuated athlete.
1966		Trimble 5000 Adult Sex Words and Phrases 13: a.k. (Vulg.) n. An Alter Kocker or aging Lecher.
1989		H. Rawson Dict. of Invective (1991) 276: old cocker (or gaffer or fart). An old man, especially a silly or senile one; often as alter kocker [...] frequently abbreviated to A.K.
2003		Jewish Journal of L.A. 24 Jan. 🌐 Archerd has no thought of retiring. ‘You see,’ he says before hurrying off to his office, ‘I’m not such an A.K. [alter-kacker] after all.’.