Green’s Dictionary of Slang

muscle adj.

(US) physically violent.

1938		W. Winchell On Broadway 17 Nov. [synd. col.] He’s fed up with dishing muscle stuff for Jack Holt and yearns to reel off a little work of truth and beauty.
1956		‘Ed Lacy’ Men from the Boys (1967) 75: There’s nothing I’d like better than to get that muscle rat.
1969		M. Puzo Godfather 86: I know you’re not in the muscle end of the Family.
1981		A.K. Shulman On the Stroll 170: Pimpin’s a mind game, not a muscle game.
1994		T. Willocks Green River Rising 69: [He had] built himself a small but serious drugs and muscle outfit.
2005		E. Weiner Big Boat to Bye-Bye 196: Even the toughest muscle boy looking to sap [...] you has got to think twice.

In compounds

musclebrain (n.) (also musclehead)

(Aus.) a fool.

2009		P. Temple Truth 131: Get me the SOG boss, whatever musclebrain that now is.
2009		P. Temple Truth 265: The city had thousands of black growling throbbing muscle cars driven by muscleheads in muscleshirts.

muscle job (n.)

an act of violence.

1961		‘Duncan Lee’ Castro Assassinated (2009) 160: Beating Fenton had excited him, he always got excited after a muscle job.
1990		(con. early 1950s) J. Ellroy L.A. Confidential 72: It’s a muscle job and a shooting job and a job that entails asking very few questions.