Green’s Dictionary of Slang

(US) an obnoxious, inept or suspicious-looking person.

1940		S.J. Perelman in New Yorker 14 Sept. 19: Hesitating...lest her suppliant turn out to be a geep, or a wolf....A drunken geep....He retains two geeps to...trip up his inamorata and...humiliate her [HDAS].
1950		Goldin et al. DAUL 77/2: Geepo. A person, often with the implication that he is an informer, a policeman [etc.]. [Ibid.] 293: Person, slow-witted or inept [...] geepo. [Ibid.] 294: Person, suspicious-looking [...] geepo.
1990		(con. 1950) Harper’s Mag. Mar. 71: We got this big influx of Italians from the Old Country — geeps. They’re not like us [HDAS].