Green’s Dictionary of Slang

Mount Shasta n.

In phrases

atop Mount Shasta (adj.) (also from Mount Shasta) [Mount Shasta, an extinct volcano in N. California; ? the image of the addict as one whose strength has vanished]

(US drugs) addicted to narcotics.

1936		D. Maurer ‘Argot of the Und. Narcotic Addict’ Pt 1 in AS XI:2 121/1: To be from mount shasta. To be addicted to narcotics.
1959		J.E. Schmidt Narcotics Lingo and Lore.