Purporting to be someone or something you are not on the Internet in order to gain friends, attention, partners, pity, sympathy or money. Named after the faux-documentary "Catfish", Catfishers have been known to post fake pictures of themselves (including but not limited to catalog images), claim to have a dying fiance or mother, or create an entirely fictional version of themselves.
Uh oh, did you see that perfectly posed photo of Donna she posted? That girl is so catfishin'.
Oh man, I heard Northlad's girlfriend is really sick. I sure hope he isn't catfishin' us.