Green’s Dictionary of Slang

(drugs) to withdraw the plunger of the syringe, before an injection into the vein, to make sure that the needle is in a vein by sucking up some blood.

1959		J.E. Schmidt Narcotics Lingo and Lore.
1967		J.B. Williams Narcotics and Hallucinogens.
1971		Spears Sl. and Jargon of Drugs and Drink (1986).
1981		A.V. Lloyd Drug Abuse.
2001		ONDCP Street Terms 2: Backtrack — Allow blood to flow back into a needle during injection.