Green’s Dictionary of Slang

(US Und.) bonus money, extras.

[1859		Hotten Dict. of Modern Sl. etc. 98: spiffs the percentage allowed by drapers to their young men when they effect sale of old fashioned or undesirable stock].
1900		‘The Wayback Family’ in Sun. Times (Sydney) 16 Dec. 5/1: The ‘nice’ young man rubbed his hands secretly at the many ‘spiffs’ he was scoring for himself by working off all the old goods in stock.
1934		T. Minehan Boy and Girl Tramps of America (1976) 84: When I see a woman going home I follow her and hit her for some clothes and lots of times I get a big dinner, and some spiffs too.
1949		Monteleone Criminal Sl. (rev. edn).