Green’s Dictionary of Slang

a general excl. of dismissal; a euph. for fuck! excl. (1)

1896		G.F. Northall Warwickshire Word-Book 80: Fish! Expressive of contempt, disparagement [...] It is a euphemism, I believe, for a slang term for pudenda. ‘Fish! to it.’.
1999		Ian Dury ‘Jack Shit George’ 🎵 Don’t you feel ready to conquer the globe? / Oh fish.
2001		Indep. Rev. 3 July 1: There’s the decaffeinated version: ‘Crikey!’ ‘Fish!’ ‘Shivers!’.