Definition of nittiest:
(adj) :
(Excessively) detailed or specific; fastidious, fussy, nit-picky.
(adj) :
(chiefly British, slang) Foolish, inane.
(n) :
(African-American Vernacular, MLE, slang) A dope fiend, a druggie.
(adj) :
(poker slang) Of a poker player: playing in an overly cautious and reactive manner.
(adj) :
(archaic, also figuratively) Full of nits.
(adj) :
(obsolete, rare) Shining; elegant, spruce.