When you are completely wasted/high stoned to the point of totally losing it. Can only really reach "pelican fly" on cocaine, amphetamines. E, LSD. But some people can reach it on marijuana.
Origins from the film "Scarface." Al Paccino is sitting in a bath watching TV and sees a pair of flamingoes flying and says : "Hey Manny look at it .. Pelican fly come on pelican."