Strange in a dreamy, unreal way — the way things look or feel to someone on a drug like LSD. Melting colours, patterns that seem to move, sounds that stretch, time going soft. People use it most about art, music, films, and videos: "a trippy album cover", "the ending was really trippy". It is casual, spoken English, and it is usually a good thing, or at least an interesting thing — it means fascinating and strange, not frightening. Here is what matters for using it: most people who say "trippy" today have never taken a drug. The word has drifted. It now covers anything that bends your sense of what is real — a dream you cannot shake, a hall of mirrors, a coincidence that is too neat. "That's so trippy" can just mean "that's wild, that's hard to get my head around". Because of where it comes from, it still sounds young and informal, so keep it out of school essays and formal writing.