Green’s Dictionary of Slang

1. nonsense, rubbish, lies.

1992

R.C. Cruz Straight Outta Compton 17: Hitting us with all kinds of yacky-yack/mucty-muck.

2. aimless, aggressive talk.

1974		R.D. Abrahams ‘Black Talking on the Streets’ in Bauman & Sherzer Ethnography of Speaking 254: [A]ny of the terms for signifying may be used negatively, like talking shit (now with an emphasis on the last word) or woofing. Older terms are spouting and muckty muck (Major 1970) and boogerbooing or beating your gums (Hurston 1942).
1994		(con. 1950s–90s) C. Major Juba to Jive.

3. see muck-a-muck n. (2)