Green’s Dictionary of Slang

professional.

1904		Salt Lake Trib. (UT) 10 July 12/1: He has [...] hopes of winning the world’s amateur record for under water swimming before turning ‘pro’.
1916		Day Book (Chicago) 17 Feb. 10/1: Oscar Mathieson of Norway, world’s champion amateur ice-skater will turn pro.
1925		Dover Exp. 27 Mar. 15/4: Some local supporters are urging us to go ‘Pro’ and enter Division 1 of the Kent league. [...] We will not go ‘Pro.’ The financial staus of tthe Kent professional clubs is not a good recommendation.
1934		(con. 1920s) J.T. Farrell Young Manhood in Studs Lonigan (1936) 246: He saw himself flashing through that semi-pro circuit like a comet.
1947		N. Algren Neon Wilderness (1986) 207: A scarred black bag [...] the one he had carried to his first pro fight.
1956		H. Gold Man Who Was Not With It (1965) 285: She used to work for Nancy as a pro exhibit-it.
1965		G. Melly Owning Up (1974) 52: Later on, when we went pro, we began to lose touch with life in London.
1974		R. Blount About Three Bricks Shy of a Load 118: Playing pro ball, he said, was also a grind.
1981		J. Ellroy Brown’s Requiem 109: The Gaffany dame was a semipro b-girl.
1993		M.B. ‘Chopper’ Read How to Shoot Friends 45: A former pro boxer and nightclub bouncer.
2001		(con. 1964–8) J. Ellroy Cold Six Thousand 19: What about Rogers and the pro shooter?
2013		I Kirkman ‘Cocaine Starlight’ in ThugLit Nov.-Dec. [ebook] Joseph left art school to go pro.