Green’s Dictionary of Slang

messy adj.¹

(also mezzy) immoral, unethical.

1940		E.F. Frazier Negro Youth 142: ‘Negroes who try to pass for white are ‘messy’ anyway. They usually think they’re so much better than other people’.
1941		‘Digg Mee’ ‘Observation Post’ in N.Y. Age 17 May 9/7: Hank Boyd got wise to that Bronxville chick and all her guys. [...] He thinks that Inezzy is powahful ‘mezzy’ .
1944		D. Burley Orig. Hbk of Harlem Jive 126: I feel sorta messy, just as if I’d blow my top if I didn’t lay this heavy scribe on you.
1963		(con. 1919) E. Asinof Eight Men Out 13: Was there something ‘messy’ in a ballplayer’s life that could be held over his head?

SE in slang uses

In compounds

In phrases

get messy (v.) [euph.]

1. to act in unrestrained manner, poss. sexually.

1915

S. Ford Shorty McCabe on the Job 60: ‘What you got, Jim?’ puffs one. ‘Young hick that got messy in the tango joint,’ says Jim.

2. (US) to have sexual intercourse.

1954

J.D. MacDonald All These Condemned (2001) 169: He had to start leering at me in that way he has. I told him not to get messy. Honestly, he wants to get messy at the darnedest times. There’s never any buildup. He just looks at you and boom.