Green’s Dictionary of Slang

(US drugs) incapacitated from an excess of a drug, marijuana or cocaine.

1917		F. Packard Adventures of Jimmie Dale (1918) II xi: ‘Youse can take it from me, Larry, dat if youse gets whiffed’ — he swore threateningly.
1958		W. Talsman Gaudy Image (1966) 155: She loved him more when he was whiffed than when he wasn’t. When he had a head full of weed, he nearly drove her wild crazy.