Green’s Dictionary of Slang

(Aus./US und.) a fool, a (potential) victim.

1895		Mirror of Life 27 July 14/2: And this ugmer soon was spotted / By the gonophs in the crowd.
1923		Smith’s Wkly (Sydney) 11 Aug. 15/4: The ugma shouted fer the mob and bridges a roll thick enough to choke Big Mouth Annie.