Green’s Dictionary of Slang

gazlon n.

[Yid. gozlin, a swindler, an unethical person]

(UK Und.) a small-time, poss. timid, thief.

[UK]D. Powis Signs of Crime 185: Gazlon Sneak thief, especially a timid thief. One big-time thief might use it as an insulting term to a rival. A Yiddish term.