An initialism of "lysergic acid diethylamide," commonly referred to simply as "acid," a hallucinogenic drug that alters one's thoughts or emotions or causes one to see or hear things that are not real. Somebody spiked his drink with LSD, and he spent the rest of the night tripping out in the corner of the room. One time, I took so much LSD that I hallucinated an entire conversation with a friend who wasn't even in the same room with me.