Green’s Dictionary of Slang

(US black/prison) psychotic; eccentric.

1997		E. Little Another Day in Paradise 215: Only reason I ain’t fifty-one-fifty is you been holdin’ me all night.
2001		(con. 1975–6) E. Little Steel Toes 79: He looks all the way 51/50, certifiably insane. [Ibid.] 109: I hope she don’t find it out ’cause she’d surely go fifty-one/fifty.