Green’s Dictionary of Slang

gonnie n.1

[Angoni (Nyasaland) goni, to stab]

(S.Afr.) a knife.

[SA] A. Brink Dry White Season 80: ‘My wife keeps on nagging me to stop before someone tries to pass me with a gonnie’ – making a stabbing gesture with his left hand to clarify the tsotsi expressions he seemed to relish.