Perma-grin
An automatic and irremovable smile that appears on the face of people who are thoroughly intoxicated with some narcotic; usually marijuana, hashish, mushrooms, or lsd.  Can be found in other cases such as on television morning news anchors, beauty pageant contestants, pop queens, and mental patients.
"Look at Greg!  Dude has Perma-grin!  That must be some good shit!"