The act of being an immature, foulmouthed, idiot on a forum, in a chatroom or on an online game chat system simply because you're anonymous, you have a (relatively) captive audience, and because you can

It’s an acronym standing for Can’t Remember A Fucking Thing. It’s usually used to describe embarrassment at being unable to remember someone (CRAFT moment); going onstage as the lead in the latest theatrical performance and your mind goes completely blank (CRAFT situation); the next morning after a night when you got pissed as a fart and woke up in bed cuddling a traffic cone and a string of fairy lights wound round your prick.