Green’s Dictionary of Slang

1. a dandy.

1873		Sl. Dict. 303: Spiff a well-dressed man, a ‘swell’.
1909		Eve. Star (Wash., DC) 11 Sept. 20/2: A film was showing a spiff with the booze willies.
1929		M. Levin Reporter 339: The reporter expected the peppy young spiff any minute to [...] make a thumb-to-nose salute.
1940		M. Marples Public School Slang 6: A spiff was a general slang term for a well-dressed man.
1977		(con. 1949) J.G. Dunne True Confessions (1979) 323: It’s one o those flowers the spiffs used to wear in their buttonholes.

2. (also spiffer) something first-rate, exciting, stimulating.

1885		‘’Arry on ’onesty’ in Punch 31 Jan. 60/1: The ’igh-flying crickits may splutter, the sleek soapboard crawlers may sniff / But gumptioners know that wot pays is the pink and the spicily spiff.
1900		Carse & Shand [perf. Ernest Shand] ‘Consequences’ 🎵 Tommy thought, ‘By Jove, / This pond's a spiffer’.

3. (Aus.) a small amount of money.

1910

Truth (Perth) 1 Oct. 4/7: The ‘finger’ who will get you / ‘On a string’ / Is a ‘josser’ who has likely / ‘Had a fling;’ / His ‘skyrockets’ then are ‘stiff’ / And he hasn’t got a ‘spiff’ / And he wants to ask you if / You can ‘spring.’ .