The instance when a hippie-type is extremely excited.
Originated from the fact that hippie's are known for wearing the shoe brand BIRKENSTOCK; and "Stoked" is surfer/stoner slang for "excited"
HIPPIE 1: "DID YOU GET ANY WEED LAST NIGHT!?
HIPPIE 2: "OH MAN, I GOT 4 GRAMS, I'M SO BIRKENSTOKED!"