Uncle Benny, n. — Green’s Dictionary of Slang

Uncle Benny n.

also Uncle Ben

[typical Jewish name Benjamin; pawnbroking is seen as a ‘Jewish’ occupation]

(US Und.) a pawnbroker.

1929		E. Booth Stealing Through Life 70: Fred preceded us into the shop. The gun was laid upon the counter [...] Uncle Ben explained at great length on the merit of the weapon.
1949		Monteleone Criminal Sl. (rev. edn).