To withhold all information concerning one's actions while in a state of complete intoxication (similar to that which one would be in after consuming a fifth of liquor). Such withholding is an effort to protect one's reputation, but also could be due to the fact that the individual simply drank too much to remember what happened during the time in question.
Dan: "Dude, did you *seriously* hook up with Sandra last night? I thought you hated that bitch."
Steve: "I can't even remember what happened after we left the bar. I'm gonna have to plead the fifth on that one."