Green’s Dictionary of Slang

(US) an eccentric person.

1918		H. Simon ‘Prison Dict.’ in AS VIII:3 (1933) 33/1: ZIB. A nondescript nincompoop. If a man was neither a hard guy nor a political, and if it seemed that he might be a dumb-bell, he was a zib. [Possibly this derived, deviously, from GAZEBO.].
1923		Wodehouse Inimitable Jeeves 181: It seemed impossible to rouse the poor zib to a sense of his position.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 244/1: Zib. (Leavenworth Federal Prison) A brainless dolt; a fool.