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1. slang A nickname for the psychedelic drug lysergic acid diethylamide (LSD). I had a real bad trip on mellow yellow last time—I'll never do drugs again.
2. slang Dried banana peel fibers for smoking. (Smoking them was thought to produce effects similar to LSD). At first, I thought my parents were telling me that they dropped acid back in the day when they mentioned mellow yellow, but now I know they were talking about some weird thing involving banana peels.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.