I always thought this term referred to someone who couldn't figure out how to win. I'm not sure where I get that from it just makes sense to me, I'm not sure about all the other definitions in here but they seem like something else other than loser to me.
I would say a loser is someone who is never on top, someone who makes bad decisions and comes out on the wrong end of things as a result.
take the guy in the photo with the pink polo that has a collar popped. We don't know anything about him except his bad decision to wear that shirt makes him appear stupid, a loser.