Green’s Dictionary of Slang

(US drugs) used between a dealer and addict to indicate that the drugs are on their way.

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 126/1: sizendizup. Peddler-addict argot, impossible to translate literally; the general import is that the coast is clear and the sale may proceed.
1959		J.E. Schmidt Narcotics Lingo and Lore.