1. To be high on marijuana. (see example 1)
2. To be unusual to the extreme, and/or to do outlandish and abnormal things. "Rareness" is often characterized by a disregard for social norms. Rare is somewhat related to its literal definition, being uncommon, but with a positive connotation. (see examples 2)
1. I knew "the rick" was rare at lunch today, because he went right for the ice-cream after entering the dining hall with bloodshot eyes and a huge grin on his face.
2. It was rare when Merk informed us that he planned to enjoy some "superior drinkabilities" (bud lights) during halftime at his son's soccer match.
The way that percussionist shook his gourd was rare.
Mr. G's tendency to show up completely pie-eyed (drunk) to formal events is very rare.