Someone who knows everything there is to know about sports but gets almost no physical activity in his/her life; their main past time is playing in fantasy sports pools on the internet.
Normal human being: "Dude, wanna go do some midget tossing?"
Fantathalete:"No way man, I've got to catch up on my fantasy pools. I've got a draft starting in 25 minutes."
Normal human being: "Dude, we're playing against hot Las Vegas strippers and they'll be naked."
Fantathalete: "Sorry dude; if i don't start the draft on time the world around me will crumble."