A "Squishy Hippo" is a term used to describe a person who is disguisingly friendly but secretly a bitch who may be hot but is a slutwhore or a player with a really wide vagina.
"Squishy" is a name to describe the hidden bitch within.
The name "Hippo" comes from the hiding characteristic of a Hippopotamus, being able to hide their anger by being calm and/or lazy.
Friend: I'll see you later, Gurl.
Squishy Hippo:... OK, Gurl. Love ya bye!
(Friend exits the room)
Squishy Hippo: Oh god I hate her.
Jack: Dude I wanna fuck that chick.
John: Maybe, dude. But she's a Squishy Hippo!