Green’s Dictionary of Slang

(US) veteran, experienced.

1939		W. Winchell On Broadway 7 Dec. [synd. col.] At 50th and Broadway, three vet newspaper gals – The Corby Sisters.
a.1954		‘Ball of the Freaks’ in D. Wepman et al. Life (1976) 112: Three snorts of that air made vet junkies kick.