Green’s Dictionary of Slang

1. (US) to defeat thoroughly.

1894

S. Crane in N.Y. Press Nov. in Stallman (1966) 106: Well, they monkeyed with the band wagon and they got slumped. Good job.

2. to murder, to kill.

2012		Eve. Standard 20 Apr. 9/4: [She] bought a block of knives [...] before the attack and told friends that ‘if they see anyone they’re gonna get slumped’.
2012		‘Dutch’ ? (Pronounced Que) [ebook] ‘Slump ’em,’ said Drama into the hpone [...] Loud gunshots rang out.
2020		G. Krauze What They Was 106: This brer from SK [...] got slumped in the middle of a rave in Scala in King’s Cross.