It means someone who gets 'lucky'. It is used mostly in situation where a person, considering themselves to be the best on the server (or at least better than the player he just got wasted by) says "Lucker!!!!". It can also be used as an incorrect replacement for "Lucky".
Ph33r : lol lucker
*1337 killed Phe33r with USP*
Ph33r : lol u got lucker
Person 2 "What?! What just happened? What did you do?"
Person 1 "I JUST HIT $100 ON THIS SCRATCH TICKET!!!"
Person 2 "WOW! Luckers!"
also Flucky which is a lucky fucker