mop!, excl. — Green’s Dictionary of Slang

mop! excl.

[jazz use mop, the last beat at the end of a jazz number with a cadence of triplets]

(orig. US) a word used to indicate a sudden occurrence, e.g. I’m doing this, then mop! I’m doing that.

1942		D. Burley N.Y. Amsterdam Star-News 17 Oct. 10: No more 13-inch pegs. Mop! No more two-inch cuffs. Two mops! [...] No more slashed pockets to push your deuce of grabbers into. Mop! Mop! Mop!
1953		L. Durst Jives of Dr. Hepcat (1989) 2: Baby you got to elucidate, I don’t know the happenings, all at once, mop! I get the ‘ice berg act.’ So if you just pad my knowledge box I can make everything alreet.