means A thick, blended drink made from fruit, juice, yogurt, or similar ingredients pureed until smooth.

from Built straight from the adjective "smooth" — itself a very old English word, going back to Old English "smoth," describing surfaces with no bumps. The drink sense is American, emerging in the 20th century as a way to name a beverage prized for its even, lump-free texture; "smoothie" had earlier slang lives too, meaning a suavely charming person, riding on the same idea of frictionless smoothness.