Green’s Dictionary of Slang

1. (US) to crash.

1942

C.R. Bond 28 Apr. in A Flying Tiger’s Diary (1984) 160: We fear he may have mushed into the lake on a gunnery dive.

2. (US black/P.R.) to beat up, to hit.

1995		(con. 1985–90) P. Bourjois In Search of Respect 237: I went, ‘Fucking Bitch!’ [swinging both fists] And I mushed her. I was pissed, man.
1997		T. Fontana ‘The Routine’Oz ser. 1 ep. 1 [TV script] I mushed his ass.