Definition of unlucky
Literally, unlucky. A player is said to be unlucky (or "UL" in an online chat room) if they lose a big hand when they were favored. There are many ways to be unlucky. If you are at the final table of a prestigious tournament and you push your last chips with AA and lose the all in to a player who would have called your all in with 22, then you are truly unlucky.
You will be unlucky one day
If you play poker on a regular basis, you have to get used to the idea that you will be unlucky at some point. When you play a lot of hands, even if you have a 1 in 100 chance of a particular badbeat or suckout, it's inevitable that you will. When you play a lot, you can play up to 5,000 hands of poker a day. That's 150,000 hands a month! Even the worst badbeat that happens once in 10,000 hands will happen to you 15 times in your month! Let's keep things in perspective.