mezz, adj. — Green’s Dictionary of Slang

mezz adj.

[the positive reputation of ‘Mezz’ Mezzrow (see mezz n.)]

(US black) honest, dependable.

1944		Cab Calloway New Hepsters Dict. in Calloway (1976) 258: mezz (n.): anything supreme, genuine. Ex., ‘This is really the mezz.’.
1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.