1. In traditional fighting videogames, a combination of attacks 1337 or expert players perform in order to frustrate, annoy, or intimidate their opponent. These combos can range from 10 hits to 10 seconds and/or deplete anywhere from 40%-60% of the victim's health bar. It doesn't necessarily label the performer a "no life" as any casual gamer can excecute a no-life combo as well with a little practice.
2. Over time, the term grew on to expert players and so they began to use it positively, for the skilled are proud of their combo mastery.
P1: *throws an insane seemingly endless combo on P2*
P2: "DUDE HOW THE F*CK AM I SUPPOSED TO STOP THAT!!? I MEAN JUST LOOK AT THAT NO-LIFE COMBO"
P1: *Grins in satisfaction*
Mr. 1337: Get ready, bitches. I'm about to unleash some/all of my world famous no-life combo(s) on that ass.