A 'butterface hottie' is a woman who has an excellent BODY (tits, ass, legs whatever it may be) but a 'suspect' face (you take away the body and she would likely not be attractive). HOWEVER, the face cannot be such a negative that it takes away from the attractiveness.
So eg. a girl with a not-so-pretty face but great body (butterface hottie) is different from a girl who is downright unattractive facially even with a great body (NOT a butterface hottie)
PS: DO NOT take any offense to being labelled a 'butterface hottie'. It means you're still hella sexy and the truth of the matter is simply this - You can't all be pretty but you can still be scorching hot and cause erections everywhere you go