Too Long, Didn't Read. Usually used to sum up a long story, i.e.: Blah blah blah blahsdfsdfsdjfvisdfgbaisdfguisdouvdifu
I once saw it as a reply to someone's long post. As in
OP: Blah blah blah blah blah blah blah-bitty blah
First response (by different poster): TL;DR
I thought it was breathtakingly rude. If you don't want to read a long post, fine, but no need to share.
I'm normally fairly tolerant of sayings, but YOLO drives me nuts simply because it is so misused.