Green’s Dictionary of Slang

1. (UK juv.) a stupid mistake or situation.

1953		A. Buckeridge Jennings’ Diary 245: It was just a sort of accidental bish – er – a mistake, I should say.
2017		M. Hyde in Guardian 5 May 🌐 The daft voters, who will probably go and make another massive bish on 8 June.

2. (US teen) a fool.

1963

Kingsport Times-News (TN) 24 Nov. 82/1: [A] ‘bish’ or goopus (both terms mean ‘dope’).