Green’s Dictionary of Slang

(Irish) an unexpected blow, delivered from behind; thus as v., to betray.

1969		P. Boyle All Looks Yellow to the Jaundiced Eye 55: ‘Someone gave me a belt of a fist,’ he says. ‘When my back was turned. Gave me a bloody judas.’.
1964-69		S. James in Calt I’d Rather Be the Devil (1994) 285: ‘I can look at a woman and tell just about what type of girl she is . . . what she’ll stand, and what she want, an’ whether she’ll ‘Judas’ or not’.
2021		P Howard Braywatch 18: ‘The doorty Judasing fooks’.