"to talk, to chatter," slang, 1950, probably echoic (cf. Australian slang yacker "talk, conversation," 1882). Yackety is recorded from 1953.
Slang definitions & phrases for yack
yak
noun
Talk, esp idle or empty chatter; mere babbling: All they can talk about, yack-yack-yack, is their own specialty/ I don't care how owlish you look, how convincing you sound, this is just yak yak yak until you do it/ in the midst of all the political yuk-yuk that dins around us/ if the State Department would stop its incessant yakitty-yak(1958+)
A laugh; a guffaw: ''Take off your clothes.'' Pause for audience yuks/ It makes me furious when I have a corny line and it gets a yock(1938+)
verb
(also yack it up or yak it up or yock it up or yuk it up): Everybody is yakking out an opinion on whether he should now reconsider his candidacy/ sparing the rod and yak-yakking and explaining all the time/ The students were seated on the floor, still yocking away/ I'll be 75 and hanging around bars yocking it up(1950+)
(also yack it up or yak it up or yock it up or yuk it up): Ken Gaul is yukking, tugging at his pointy satyr's beard/ There'd be Don, yockin' it up like crazy. He's so hysterical with laughter/ former senator George McGovern, yukking it up with Paul Volcker(1938+)