Green’s Dictionary of Slang

1. a man wearing a frock-coat; a frock-coat.

1909		‘O. Henry’ Roads of Destiny 261: His long-tailed ‘frock’ made him not the least imposing of the official family.
1970		(ref. to WWI) L. Hart Hist. of WWI in DSUE (1984).

2. (US Und.) a suit of clothes.

1929		C.G. Givens ‘Chatter of Guns’ in Sat. Eve. Post 13 Apr.; list extracted in AS VI:2 (1930) 132: frock, n. Suit of clothes.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 74/2: Frock. A suit of clothes.
1983		R. Aven-Bray Ridgey-Didge Oz Jack Lang 28: Frock Clothing.

3. (US gay) the man who poses as a lesbian’s ‘husband’ for the sake of ‘passing’ in an intolerant society [the image of the lesbian as a trouser-wearing woman for whom a frock is automatically unnatural].

1997

Roger’s Profanisaurus in Viz 87 Dec. n.p.: frock n. Of lavender marriages, a cosmetic husband. The lesbian’s equivalent of a beard (qv).