Green’s Dictionary of Slang

(US gay) to practise cunnilingus.

1932		‘R. Scully’ Scarlet Pansy 254: ‘Mr and Miss Fish!’ ‘Again I’m bothered,’ said Miss Savoy. ‘Noun or verb?’ ‘Verb, of course, stupid,’ explained Fay. ‘Active or passive,’ chortled Miss Savoy.
1937		S. Kahn Mentality & Homosexuality 184: Many fags have sexual relations with women, (‘Fishing’) means to go down on them’ [Simes:DLSS].
1941		G. Legman ‘Lang. Homosexuality’ in Henry Sex Variants II 1165: fish [...] Employed as a verb, usually intransitive, meaning to perform cunnilinctus.
1972		B. Rodgers Queens’ Vernacular 81: fish [...] 6. (v) to muff dive. Syn: chew [eat] fish.