Green’s Dictionary of Slang

(N.Z. prison) an inmate who sees themself as more important, influential than they actually are.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 97/1: kackle pants n. a person who mistakenly regards himself as being in a position of power; the opposite of a king pin: ‘A person who thinks he’s the KP when he’s not [...] KP abbr. 1 = kackle pants.