Green’s Dictionary of Slang

(US) a general pej.

1920		C. Sandburg ‘Cahoots’ in Smoke and Steel 45: Go fifty-fifty. / If they nail you call in a mouthpiece. / Fix it, you gazump, you slant-head, fix it.
1944		N. Davis Sally’s in the Alley 111: ‘The head gazump, you mean? What’s the matter with him?’ ‘He claims he’s God.’.