To falsely assume something about a person. Often based on stereotypes or outward appearances. It can be used in both the positive and the negative (ie, someone is "pegged" with a characteristic they do not possess; or they are "not pegged" with a true but hidden characteristic.)
Derived from the hypothetical idea of "pinning (or pegging) a label on someone".
"I never would have pegged Ben for a geek, but then I discovered his collection of Star-Wars action figures."
"I'm sick of people pegging me for a homosexual. There's nothing wrong with a little judicious manscaping here and there!"