Froob is a combination of three words, for use on people who deserve to be called froobs. The words are (the F word) + (Rube*) + (Noob). "F" from the f word, R from Rube, OOB from Noob.
A Roob is worse than a Noob, but a Froob is the king of the dung heap.
*For those of you who don't know, a "rube" was a sucker. Rubes were often picked out by scam artists of olden days.