Green’s Dictionary of Slang

Y.M n.

[abbr.]

(orig. US) the Young Men’s Christian Association; also attrib.

[US]S. Lewis Main Street (1921) 257: I wish we had a Y.M.
[Aus]F. Grose A Rough Y.M. Bloke 43: They brought him along to the Y.M. Dug-out and gave him a cup of cocoa.