Is that some sort of Yiddish slur?
I’d never heard the term before. According to something called “The Urban Dictionary:
Fench : paradoxical act of aggression - as a fench is entirely unprovoked, any retribution won’t serve as due revenge, because the revenge was provoked by the fench. No prior warning can be given to the victim, and a fench cannot be used as a threat, becase these both imply provocation of some sort - a fench must be an act of wanton, spontaneous violence.
In other words, an act of aggression so successful that full revenge is impossible.
Can be used in various forms to describe the act of aggression (usually resulting in comic humiliation of the victim), the aggressor and the victim, ie:
1. “I’ve just witnessed a particularly amusing fench”
2. “I’ve just been fenched”
3. “You’re worse than a b*st*rd - you’re a fencher!”