It can be used as an adjective, noun, or verb.
A fench is someone who is old, and bloated. Otherwise known as the adorable little man from "Up!" Can be used positively, negatively, and neutrally!
Noun: You're such a fench, you just ate an entire pizza!
Adjective: Don't act so fenchy and let the handicapped person use the handicapped parking spot!
Verb: Go fench youself!