Green’s Dictionary of Slang

(US) a thug.

1942		Berrey & Van den Bark Amer. Thes. Sl.
1959		W. Tevis Hustler (1998) 74: The small-time men – the scufflers, musclers, dollar jumpers – prey in nibbles.
1990		R. Campbell Sweet La-La Land (1999) 48: In case any musclers tried to rob him, all they’d find was some small change.
2001		R. Porrello To Kill the Irishman: The War That Crippled the Mafia n.p.: The murder resulted from a conflict with a well-known and successful bookmaker in Los Angeles who used Petro as a muscler.