Green’s Dictionary of Slang

1. a person, usu. a thug, used to enforce his (or another’s) will through violence or threats of violence.

1952		(con. 1927) G. Fowler Schnozzola 107: Three gun boys were on the prowl for the Quaker. The ‘enforcers’ chanced to be sitting in the club.
1952		Lait & Mortimer USA Confidential 156: Jimmy (the Weasel) Fratiano is the new enforcer.
1978		R. Caron Go-Boy! 123: Mom used him as an enforcer when Dad wasn’t around.
1999		Indep. 12 July 2: Jack Cunningham, the cabinet ‘enforcer’.
2002		J.J. Donohue Our Own Helen C 298: Did you ever think that maybe carrying two guns and a knife, and talking like an enforcer for the mob, might cut into your social life?

2. (gay) a lesbian who keeps the other women in line.

1972		B. Rodgers Queens’ Vernacular.
1986–7		Maledicta IX 150: The original argot of prostitution includes some words and phrases which have gained wider currency and some which have not […] enforcer (a dyke who keeps the girls in line).

3. (Aus.) in horse racing, a jockey who relies on the whip.

1996

J. Byrell Lairs, Urgers & Coat-Tuggers 125: [G]ood old Uncle Ern usually couldn’t cop ‘enforcers’ - jockeys who don’t mind laying in with the old persuader.