1) Nellson means someone that everyone wants to fuck.
2) Someone popular who everyone wants to consider a friend.
3) Someone who easily influences members of the other sex into doing things beneficial to themselves. (also see; pussy whipped OR owned.)
Boy 2 - "Oh yeah, she's a real Nellson"
2) "Oh my, look at Danny, he's a nellson for sure. Think he'll come to my party?"
3) Nellson - Can u do my homework craig?
Boy - I am supposed to be going to a mates birthday tonight, but i'll just stay home and do it instead.