Green’s Dictionary of Slang

(N.Z.) a terrible example of something, a bad representative of something, anything useless; usu. as she’s a stag of a...

2003

McGill Reed Dict. of N.Z. Sl. 198: stag Anything useless, eg, ‘She’s a stag of an edge-trimmer, mate, time to get a new one.’ From rural meaning of an improperly castrated ram. C20.