I plead the 5th
Pleading the 5th means to not self-incriminate yourself, but it has the connotation of meaning that you did it.
The reason for this connotation is because if you didn't do it, why wouldn't you just say no?
It's a way of not saying "yes you did something" but hinting that you did, but they can't really say you hinted at it.
Bob: Did you have sex with my girlfriend?
Joe: I plead the 5th