Green’s Dictionary of Slang

Y n.

[abbr.]

1. [late 19C+] (orig. US) the Young Men’s Christian Association (YMCA), often as the Y.

2. attrib. use of sense 1.

3. the vagina; in phrs. dine at the Y under dine v.; eat at the Y under eat v.; lunch at the lazy Y v. [punning on sense 1 + the shape of a woman with her legs spread].

4. [2000s] in pl., Y-fronts.

5. the shoulders and arms as a place from where punches are thrown; in phrs. below [? the shape of the torso with arms raised].

In phrases

go from the Y (v.) [go v. (11)]

[1970s+] (US black) to fight.