The devolution of an ethnic group that derived from the European settlers and conquistadors as they bred with the native Aztec people of central Mexico. Factor that human outcome with the addition of deliciously fried (use of manteca) Mexican foods (enchiladas, tacos, gorditas, etc) and you get the mantec person.
Skylar: What do you think that guy is? He's too fat to be an Aztec type of Mexican.
Tyler: Hmm, probably got fat from eating all that greasy Mexican food.
Skylar: Yeah, he's probably part Mantec.