Green’s Dictionary of Slang

muscleman n.

[note Hughes, Tom Brown at Oxford (1861): ‘I must call the persons in question “musclemen,” as distinguished from muscular Christians’; his ref. is to those who celebrate physical strength but have no religious faith; note 1945 Minnesota Univ. use muscle moll, a strong man]
(orig. US)

1. a thug, usu. as employed by a gangster for purposes of intimidation; any strong, aggressive man.

[US]Appleton Post-Crescent (WI) 25 Apr. 1/5: A ‘muscle man’ is one of a band of criminals who has taken to the kidnapping, extortion and robbery of persons who are themselves engaged in some illicit enterprise.
[US]J. Wilstach Under Cover Man 92: His two remaining henchmen, ‘Fry’ Patten and Pete Kely, muscle men, not thinkers.
[US]Mezzrow & Wolfe Really the Blues 21: I got my kicks out of rubbing elbows with all those bigtime gamblers and muscle men.
[US]B. Schulberg On the Waterfront (1964) 136: Johnny Friendly’s pair of muscle men hard-shouldered their way over to [...] another group.
[US]S. Longstreet Flesh Peddlers (1964) 336: One of the musclemen who watched the cashier’s window came over.
[Aus]J. Holledge Great Aust. Gamble 149: So Slater gained confidence and, enlisting a few reliable ‘muscle men’, set up his own gang.
[US]‘Red’ Rudensky Gonif 79: He [...] had been everything from a hijacker to a muscle-man.
B. Raveendran Saints and Sinners 25: One of the labourers had refused to give on his pay-day a heavy subscription to his muscleman labour leader for which he was knocked down and thrashed.

2. (also muscle freak) a man with an outstanding physique.

U. Michigan Chronicle VIII 268: Then a muscleman approached me, / M Jerry, give the rest a slashing. / But, remember, if you hit me. / You will get a deuced thrashing.
[US]Weseen Dict. Amer. Sl. 261: [Sports – Misc.] Muscleman – A wrestler.
[US]Berrey & Van den Bark Amer. Thes. Sl.
[US]‘Ed Lacy’ Best that Ever Did It (1957) 29: ‘This Franklin Andersun, was he a muscleman?’ Danny laughed [...] ‘Couldn’t lift a toothpick.’.
[US]F. Kohner Gidget Goes Hawaiian 77: The first thing that loomed into view when I arrived at Al Mona was a group of bronzed musclemen.
[US]W. Diehl Hooligans (2003) 42: He’s also a muscle freak.
K. LePage Some Day Never Comes 119: I was at the gym a few months ago, and right next to me was a huge muscleman pumping iron.