A girl who, besides having grotesque or deformed genitalia, is otherwise completely attractive. The nature of her genital abnormality may differ widely in terms of its cause, type or level of severity. Cited problems may include, but are not limited to, size, odor, unsightliness etc.
The phrasing of this term is derived from the common colloquialism: Butterface.
"Yo, man, I thought that girl was hot, until she got naked. She was a total buttervag."