Green’s Dictionary of Slang

Uncle n.¹

the USA, esp. the US armed forces or other federal/national authorities.

1849		Placer Times 1 Sept. n.p.: Two Express Lines have been established between our City and San Francisco. Our old Uncle will have to ‘stir his stumps’ else his ‘regular’ arrangements will become a dead letter [DA].
1873		D.M. Berrey MS Corr. (Huntington Lib.) 1 Sept. n.p.: It is very kind in our good Uncle to allow these hopefuls to experiment with appropriations to build this uncertain wall [DA].
1913		H.A. Franck Zone Policeman 88 23: Our worthy Uncle of the chin whiskers makes it quite plain that, while he may tolerate the mere visitor, he does not care to have him hanging around.
1926		Wood & Goddard Dict. Amer. Sl.
1934		Charleston (WV) Daily Mail 31 July 6/8: ‘Ice tong doctors’ sell ‘decks’ of drugs to ‘junkers’ when the ‘whiskers’ and ‘uncles’ (terms for federal agents) aren’t looking.
1942		Pittsburgh Courier (PA) 15 Aug. 21/2: Watch his stuff [...] Uncle, and tell your white off-springs that we are willing and can do any job as good as the next fella.
1949		N. Algren Man with the Golden Arm 275: Now you’ve pulled Uncle Sam’s whiskers ’n Uncle ain’t gonna care whether you talk goofy or straight.
1953		‘William Lee’ Junkie (1966) 98: ‘He belongs to Uncle now,’ said the captain to my wife.
1959		Murtagh & Harris Who Live In Shadow (1960) 17: He is one man sure to attract all the gazers, all the whiskers and uncles, all the narcotics agents.
1972		G.V. Higgins Friends of Eddie Coyle 14: I was thinking more [...] of you having the prosecutor tell the judge how I’ve been helping my uncle like a bastard.
1978		(con. WWII) T. Sanchez Hollywoodland (1981) 72: A serviceman’s got his regular wages from Uncle Sam, and if he don’t come back from overseas old Uncle is real generous with his war widow’s insurance.
2001		ONDCP Street Terms 22: Uncle — Federal agents.
2006		D. Winslow Winter of Frankie Machine (2007) 28: You can do about anything in this country as long as you kick up to the feds. Uncle wants his taste.

In compounds

Uncle’s orphan (n.)

(US black) a draftee.

1941

D. Burley N.Y. Amsterdam Star-News 17 May 11: Uncle’s Orphans marched in [...] nine dozen of ’em.

In phrases

duck Uncle (v.)

(US black) to avoid the draft.

1941		D. Burley N.Y. Amsterdam News 10 May 11: To duck uncle is to avoid the draft (and I don’t mean cold air).
1944		D. Burley Orig. Hbk of Harlem Jive 39: [as 1941].