Green’s Dictionary of Slang

uncle n.

1. [mid-18C+] (also mine uncle, mine uncle’s, my uncle, my uncle’s, uncle Monty Pete [i.e. ‘mount of piety', fr. Fr. mont de piété, a pawnbroker] ], Uncle Sam, uncle Tom) a pawnbroker [the avuncular help he gives ‘relatives’ in temporary financial distress].

2. [late 18C–early 19C] (also mine uncle’s) a privy.

3. [mid-19C–1960s] (US, also unkey, unky) a form of address to a black male whose name one does not know or ignores.

4. (Aus.) a money-lender.

5. [late 19C–1910s] as your uncle, oneself.

6. [20C+] a general term of address to a man; there need be neither prior acquaintance nor any form of relationship.

7. [1920s–40s] (US, also auntie) a receiver of stolen goods.

8. [1940s–70s] (US gay/prison) an older homosexual male with a taste for young men or boys.

In phrases

cry uncle (v.) (also holler..., say..., yell...) [poss. from a joke first printed 20/06/1891 in Wkly Irish Times (Dublin), then UK press: the punchline devolves upon the training of a parrot to cry ‘uncle’; given what seems an Irish origin, there may be a link to American Speech LI (1976): ‘“[U]ncle” in this expression is surely a folk etymology, and the Irish original of the word is anacol (anacal, anacul) “act of protecting; deliverance; mercy, quarter, safety”, a verbal noun from the Old Irish verb aingid “protects”’]

[1910s+] (US) to beg someone to stop an action, to surrender; also fig.

go to visit one’s uncle (v.) [euph; but note RAF jargon go uncling, to pursue a married woman; her children call the suitor ‘uncle’]

[late 18C–early 19C] to abandon one’s wife shortly after the marriage ceremony.

send to uncle’s (v.)

[19C] to pawn.

where uncle’s doodle goes (adv.)

[mid–late 19C] the vagina; thus be where uncle’s doodle goes v., to have sexual intercourse.