Butt ugly
Definition 1 of 1
Pro Tip 1/3Colloquial Usage
Used in casual conversation among friends, not in formal settings. 
I wouldn't say her outfit is butt ugly at a formal dinner, but I might tell you that in private.
Pro Tip 2/3Potentially Offensive
Can be seen as rude or harsh; use cautiously to not insult someone. 
My brother called my favorite poster butt ugly, and I took offense.
Pro Tip 3/3Emphasizes Extremes
Stresses a strong negative opinion about appearance; more than just 'ugly'. 
He wasn't just a little displeased; he thought the statue was butt ugly.