Or just your best insults that would work against really stupid people... think orcs in the hobbit

Your birthcertificare was a waste of paper

Birth certificate* my bad

I found sometimes just saying "No" like a statement throws people off and it usually works in any context. I usually try and confuse the person more than actually trying to insult them back, it's funnier to me.

"About time you gave yours a night off."

“I’ve been deeper inside her than you ever have.”

When someone asks you "What are you? 5?!" Just reply "Yeah. 5 inches deep in your mom!"

Yeah, she told me you only lasted two minutes, and bleated like a goat.