A woman of large stature or weight, who is either skanky, slutty, a bitch, or a combination of the three terms. (Hippo is to describe the fat, and cat or "pussy" to describe her skankyness.)
Guy 1:That chick's a total Hippocat dude.
Guy 2: Why?
Guy 1: She's fat and is a total bitch, man.
Guy 2: Makes sense.