slang LSD (lysergic acid diethylamide), a synthetic hallucinogenic drug. Possibly derived from a phonetic shortening of "acid" (a common slang term for LSD), with "Uncle" added for humorous effect. There were a few people passing around tabs of Uncle Sid at the concert. A: "What the heck is wrong with Tom?" B: "Oh, just tripping on Uncle Sid. He'll be fine."