The small space of time before you really start tripping on LSD or psilocybin mushrooms, during which several things happen: you get that funny feeling in your stomach, your head loses its weight, your pulse rate begins a steady climb, and you start yawning, A LOT. While not as fun as actually trippin, a pretrip can be very exciting and a happy time. Also lets you know that whatever you want or need to do before you start trippin better fucking DO IT NOW.
AB: Man I'm pretrippin like a motherfucker!
N: I feel pretty fuckin crazy.
RE: Let's go to the river!
AB and N: Hell yea!