A Mexican person who was born in the US and is completely assimilated into the US society from birth. Person usually grows up learning both English and Spanish and can speak both fluently. Essentially, it's just a way of saying a person is an American-Mexican. Calling them Tex-Mex may not apply if the person is not actually from Texas.
Girl: Who's that new guy over there?
Guy: That's Juan Carlos, he's in my math class.
Girl: He's hot. Is he Mexican?
Guy: Yeah but he's a Meximelt. He's from Colorado.