Green’s Dictionary of Slang

(US drugs) an ounce (28g) of narcotics.

1969		Smith & Gay Heroin in Perspective (1972) 106: It is called a street ounce or a vig ounce.
1972		E.M. Brecher ‘The Consumers Union Report on Licit and Illicit Drugs’ Consumer Reports Mag. Ch. 11 🌐 Sometimes it is called a street ounce or a vig ounce (vig, is an abbreviation for vigorish, which is the term used to designate the high interest on loans charged by loan sharks).