Green’s Dictionary of Slang

to chatter aimlessly; thus yackety-yacking n.

1949		I. Shulman Cry Tough! 66: ‘For Chris’sake,’ Jennie began [...] ‘can’t you stop your yackety-yacking?’.
1956		A. Ginsberg ‘Howl’ Howl and Other Poems 10: Yacketayakking screaming vomiting whispering facts and memories.
1957		Coasters ‘Yakity-Yak’ 🎵 If you don’t scrub that kitchen floor / You ain’t gonna rock and roll no more. / Yakity Yak (Don’t Talk Back).
1966		M. Rumaker Exit 3 and Other Stories 163: Sometimes I wisht I was back in Jamesburg the way you guys yackety-yackety-yack like a bunch of crabbing women.
1971		R.D. Pharr S.R.O. (1998) 327: ‘She followed me out of the house, still going yackety-yak’.
1995		M. Simpson ‘Prufrock Scoused’ Catching Up with Hist. 23: The pair ov uz yakkety yackin over ow good a shag it woz or not.