Green’s Dictionary of Slang

(US Und.) a Federal agent or agency.

1954		J. Thompson Swell-Looking Babe 72: I don’t like to [...] wait for old Uncle Whiskers to sock it to me.
1956		‘James Updyke’ [W.R. Burnett] It’s Always Four O’Clock 6: I did my time in Japan—Army I’m speaking of—and I got back without so much as a bad cold. [...] When Uncle Whiskers got through with me I took a quick trip back to Gary.
1959		J.E. Schmidt Narcotics Lingo and Lore.