Comedy lingo: The act of revising a joke to make it more vulgar and offensive. Usually involving the addition of rape, pedophelia, cancer, or any other generally offensive topic.
I was fitching up my jokes for dirty joke night at the comedy zone.
We brought new writers in because the script needs some fitching up.
That joke is a too clean for this crowd, you should fitch it up a little.
Taken from "Monty Python and the Holy Grail", to fetch the cow is to react disproportionately and inappropriately over a very small insult or slight. Can also be shortened to simply "fetching".
Man 1: Your scarf's quite gay.
Man 2: Say's you, fucking homo, I hate you. Your mum's sucking my cock every night, and I'm wearing this fucking scarf when she does it.
Man 1: Mate, stop fetching the cow.