Green’s Dictionary of Slang

banjax v.

also bandjax
[? Dublin sl.]

(chiefly Irish) to batter, to destroy, to ruin, to get in the way of.

[Ire](con. 1890–1910) ‘Flann O’Brien’ Hard Life (1962) 84: Some people at one time thought they were trying to banjax and bewilder the One, Holy and Apostolic.
[Ire]C. Brown Down All the Days 94: Get up till I bandjax the living daylights out of you!
[US]New Yorker 28 Oct. 40: So she ups and banjaxed the old man one night with a broken spade handle.
[Ire]F. Mac Anna Last of the High Kings 119: I blame the Brits. They banjaxed our transport service with their imperialism.

In derivatives

banjaxed (adj.) (also bandjaxed)

broken, ruined, smashed up.

[Ire]‘Flann O’Brien’ At Swim-Two-Birds 240: Here is his black heart sitting there [...] in the middle of the pulp of his banjaxed corpse.
[Ire]‘Flann O’Brien’ Third Policeman (1974) 49: The brother’s valve is banjaxed.
[Ire]S. Beckett Waiting for Godot Act II: Lucky might get going all of a sudden. Then we’d be banjaxed.
[UK]P. Boyle All Looks Yellow to the Jaundiced Eye 114: I’m properly banjaxed. Whacked out.
[Ire]J. Morrow Confessions of Proinsias O’Toole 112: Things tend to get a bit bandjaxed from now on.
[Ire]F. Mac Anna Ship Inspector 110: ‘The country’s banjaxed,’ the woman said. ‘We’re a banana republic without the bananas.’.
[Ire]P. Howard Miseducation of Ross O’Carroll-Kelly (2004) 245: Unfortunately, the equipment I’ve here with me is a bit banjaxed.
Blue Pages (Dublin) ‘Dublin Dictionary’ [Internet] Bandjaxed Broken, useless, tired.
[UK] I. Rankin Naming of the Dead (2007) 485: Ironically [...] given the amount of painkillers he’d scarfed, the first thing he complained of was a thumping headache.