A woman weighing in excess of 200 pounds.
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.