Stateotyping is when someone assumes something once you acknowledge where you might be from. This common mistake is generally made with college students.
It is better understood by looking at the two words that first melded into it.
State - another word for a territory occupied by one of the constituent administrative districts of a nation
Stereotyping - A conventional or formulaic conception or image of something; "regional stereotypes have been part of America since its founding
Person 1: Hey where did you say your from again?
Person 2: Oh I'm from New York.
Person 1: The City?
Person 2: No...Upstate New York there are more places then the city you know
Person 1 now kinda looks like an idiot.
Person 2 assures them its just a Stateotype
Person 1: You said your from Maryland right?
Person 2: Yeah how nice of you to remember.
Person 1: So your right by the beach right?
Person 2: No I actually live like two hours from the beach; Not everyone lives on or right by the beaches of Maryland
Person 1 again looks like an idiot in this scenario
Person 2 humbley says, "Your Stateotyping me I see"