Green’s Dictionary of Slang

1. (US) to kiss and cuddle; also as mush it up.

1916		Eve. Public Ledger (Phila., PA) 2 June 11/5: It isn’t romance [...] just plain, common ‘mushing’.
		Transcript Foster Inq. in L.R. Murphy Perverts by Official Order (1989) 27: Kreisberg began ‘to mush me up’ and carry on ‘as if I was a girl’.
1928		J. March Wild Party 49: He’s mushin’ it up with your angel-face!
1939		H. Miller Tropic of Capricorn (1964) 104: I caught him red-handed one night, in the movies, mushing it up with a young girl.
1949		H. Miller Sexus (1969) 57: We stopped under a tree and started to mush it up.
1953		S. Bellow Augie March (1996) 86: The kind [of girls] who’d never grab you by the fly or let you stick around till one a.m. mushing with them on the steps.

2. (US) to court a woman in a sentimental manner, to ‘chat up’.

1934

(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 394: Dapper Dan was still mushing around Sally.

3. (US black) to kiss.

1944		‘Jiver’s Bible’ in D. Burley Orig. Hbk of Harlem Jive.
1946		Mezzrow & Wolfe Really the Blues 198: Ten minutes after I made home-sweet-home [...] I mushed [my old lady] and cut out for the Riverside Towers, on the West Side.