Green’s Dictionary of Slang

of a (usu. gay) man, openly and extremely effeminate.

1974		G.F. Newman You Flash Bastard 104: Although Roger Dawes, if bent – as Sneed was sure he was – could pass for straight, not so the flatmate, Perry Lewis. He was a swishing poof, one of those who were openly proud of their effeminacy.
1996		L. Pettiway Honey, Honey, Miss Thang 201: I learned how to swish. So I was like swishing, honey.
2019		P. Baker Fabulosa 298/2: swishing flamboyantly grand or camp.