What don't I get?
He was making a joke to cover up his mistake by treating "an jerk" as a noun. In that context, it would be seen as your "an jerk", similar to your room. Get it?
I do a similar thing in the following exchange which I have:
"How are you?"
"I'm doing good"
"You're doing well, not good"
"No, I am doing good. I was using good as a substantive adjective, as in I am doing good things."