This phrase is a gag phrase on the user. Groups of people will "plant" the phrase in hopes that the unknowing user will prank themselves. The word itself does not actually have any meaning, the stigma is in not knowing that it does not have a meaning. This word is unique as it does not appear to have any true synonyms. It's a verbal example of The Emperor's New Clothes.
Hey, Sally said, "Greg is a real dorkoid." She's such an idiot!
G: Hey Lisa, I hear that chicka you're dating is a total nympho!
L: Nah, Tina talks about sex all the time, but when the lights go out, she's kinda tight about it.
G: What you got there, my friend, is a nymphoid!