Green’s Dictionary of Slang

mint n.2

[numerous advertisements promoting products with ‘a hint of mint’; note earlier minty n.]

(US gay) effeminacy; usu. in phr. a hint of mint, a trace of homosexual tendencies.

H. Raucher A Glimpse of Tiger 201: He modeled his outfit, with a hint of mint. ‘Oh — just something I stole from a Bowery mission on the recommendation of my seeing-eye dog. Do you like it?’.
[US]B. Rodgers Queens’ Vernacular 107: Hint of mint (’50s) trace of homosexual tendencies.
[US]J. Stahl Perv (2001) 83: Oh sure. Ned can spot ’em. Ned can always pick up a hint of mint.
J.F. Healy Turnabout 111: ‘I guess I always thought there was this hint of mint about the guy.’ [...] ‘You mean he’s gay?’.
[UK]R.T. Collins White Monkey 204: What caught my eye, or rather ear, was the very distinct hint of mint in the man's voice.