Someone who is shrouded in pessimism, absolutely determined to ensure you have no enjoyment in what ever activity you intend on taking part in.
Guy "Man, I can't wait to stand in line with my mom and get Medal of Honor"
Other guy "Dude, you know it's just like other Modern Warfare games on the market, except for Ghost Recon, but no one likes Ghost Recon."
Guy "Why'd you have to be such a wet weekend?"
Other guy "Wet weekend? I've just saved you $50"
miserable bastard; sulky, self-pitying, whiney person.
You can continue to be a wet weekend your whole life, but eventually we're all going to get sick of you.