Somebody who devotes their life to one specific activity because they have absolutely nothing better to do in their spare time. One such example is somebody who plays video games more than 12 hours a day because they have nothing better to do. No activity is exempt from this though, and video games can't be singled out, this applies to sports, outdoor activities, etc...
no_lifer: You have no life. All you do is play video games.
other_guy: No, I have other things I can do. I just choose to play video games.
no_lifer: Well what about me. All I have to do with my life is play football? I'm not a no lifer. I have friends.
other_guy: Actually, you don't have a life. Just having friends and spending your time playing sports doesn't mean you automatically have a life.