Green’s Dictionary of Slang

(Aus./N.Z.) something or someone that is excellent, wonderful, approved of.

1874		‘Old Calabar’ Won in a Canter III 95: ‘She’s a trimmer in a topsail breeze — that’s what she is, and no mistake — talk of the Flying Cloud.
1930		Sun. Times (Perth) 13 Apr. 6/7: The boy had recourse to a slice of modern slang. ‘You’re a trimmer, you are!’.
1941		K. Tennant Battlers 164: Gawd! she’s a trimmer.
1951		Cusack & James Come in Spinner (1960) 243: There was a woman where I lived and she was a real trimmer.
1951		D. Stivens Jimmy Brockett 73: ‘It’s a trimmer!’ Len said.
1963–74		B. Humphries Barry McKenzie [comic strip] in Complete Barry McKenzie (1988) 79: Valda Clissold, cripes she was a real little trimmer.
1967		J. Wynnum I’m a Jack, All Right 96: How’d you like a steak of roast ‘Go’ for dinner [...] there’s a trimmer we’ve got treed.
1986		R. Beckett Dinkum Aussie Dict. 53: Trimmer: If someone is said to be ‘a little trimmer’ he, she or it has done well. Normally used in reference to a horse or a dog that has won a race, but an inanimate object such as a lottery win can be held to be a little trimmer as well.
1988		McGill Dict. of Kiwi Sl. 128/1: you’re a trimmer! a compliment or a curse; from English word ‘trimmer’, a person who literally thrashes, meaning performs vigorously.
1996		J. Byrell Lairs, Urgers & Coat-Tuggers 301: In the interests of space, time, brevity and what ends up being a trimmer of a punt year, it was deemed a good idea .
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].