Term originates from former major league baseball player Mario Mendoza's batting average (~.200 lifetime). His batting average, referred to as "the Mendoza line," was basically the cut off for mediocrity. In terms of slang, a Mendoza is a woman who meets or surpasses the 200 lb line.
This term can be positive as well as condemning. When hogging, a Mendoza is essentially a bona fide target. For any other social situaton, Mendozas are generally feared and avoided.
Guy2: "Fuck. You're going to out-Hog me."
Guy1: "I barely avoided the Mendoza at the bar last night. She would have crushed me and ruined my self-esteem."
Emphasis can be added with a gaze into the sky and a shaking of the fist.
Also referred to as giving a facial.
She is so hot, I'm going to Mendoza her tonight.
Friend B: Isn't he an Engineer or Doctor or something???
Friend A: Yeah he's a Mendoza...lucky bastard!
Simultaneous text from person B: "You can never really know too many Mexicans."
Text from person A: "mendoza!"
Text from person B: "mendoza!"