Used when referring to someone who's smile makes you feel uncomfortable. Usually a full toothed grin with widened eyes and raised eyebrows. You usually find yourself expecting them to say, "Hey kids, get in the van, I have candy for you."
Guy: You know Barney? He totally has a stalker smile.
Guy2: I know, right? I can just picture him on the news for rape charges.
Girl: Mr. (Insert creepy teacher's name here) has such a stalker smile. It creeps me out. I can't stand to be in his classes.
Girl2: Ugh me either. It always feels like he's checking me out.