slang LSD (lysergic acid diethylamide), a synthetic hallucinogenic drug. Possibly derived from a phonetic shortening of "acid" (a common slang term for LSD) to "sid," then expanding that abbreviation to match the name "Sidney." There were a few people passing around tabs of sidney at the concert. A: "What the heck is wrong with Tom?" B: "Oh, just tripping on sidney. He'll be fine."