Never heard it but dweeb suggests geek, so I'd guess it's a derogatory term used for a certain type of metal but fans of another sort or for metal in general but those who don't like it at all.
I'd assumed ir was what Rolling Stone called bands like Weezer and Wheatus where the singer has big glasses and is a weeny bit ugly, so they can do this fawning "omigod he is slightly wierd but OH so dashing and COOL!" that still manages to get on my tits to this day
Its a 'jokey' term that was generally bandied about by his friend who made fun of his passion for music from metal band with female singers.
When I was at Progpower USA a few weeks back I handed a copy of 'Dark Waters' to Chris (Dweeb), for him to pass the expert opinion on. He likes it more than me
You can also hang out and discuss 'Dweeb Metal' at Sonic Cathedral (another recipiant of a copy of 'Dark Waters', during my visit to the States)