A person who thrives on negativity. Somebody who never manages to say anything positive but can waffle on for five hours about the negative side of anything.
Commonly observed in disgruntled employees.
Prit: "Hey, Adie, we've got a 10% pay rise this year"
Adie: "That's rubbish - I wanted 15%. And a bonus. And a car parking space. And another laptop. This place is crap"
Prit: "Shut up Adie, you're such a neg ferret"