Slang for fish sandwhich, it's a situation when a fish, normally edible, gets caught inbetween two slices of bread, in extreme situations, a baguette.
Some people think that it is a fish that performs dark arts and travels by broom stick with their wicked fishy ways but they are wrong.
Customer: Yeah, I'll have a fishwhich
Shop dude: A what??
Customer: You know fish sandwhich
Shop dude: Oh I thought it was a fish that performed magic
Customer: well you're WRONG!
Moral: The customer is ALWAYS right