Filthy Pro (and its variants vile pro and dirty old pro) are generally an affectionate sobriquet for a professional writer. (While these aren't necessarily derogatory, make sure you know the pros you use it in front of and that they know your intent; see comment with Filthy Huckster.)
"These people are often called 'filthy pros' and 'dirty old pros' [or 'vile pros — because that's what they write'] because they are supposedly rich, and because it is whispered that they will stoop to any trick to do wrong to the innocent fan. The majority of them are as much fans as anyone; many are older fen who turned to writing for fun and profit [including Bob himself]. They are both despised as parasites and fawned on as minor tin ghods. And those fans who are loudest in censure are often just those who try hardest to sell fiction and thus become pros."