a suck loser is a loser's loser. there are plenty of losers, but only a few are so extreme that they are not just losers, but belong to the elite field of suck losers. a suck loser is doomed for all eternity to be a suck loser.
Jeffrey thought the woman on the L train was checking him out; in reality, she was trying to figure out how to tell him his fly was open. He is such a suck loser.
Kris thought her patient was flattering her and hitting on her. Later on the patient's family explained that he had a frontal lobe brain tumor and was speaking nonsense. That is when she realized that she is a suck loser.