They did, and I replied

"We don't know because we've never done this before, and we can only compare to the most similar work we've done, and any existing data."

Before I could say more, they I interrupted: "You have to know, it's your job, or you don't know what your doing," or something along those lines. It was the most heated I've felt. I know what I'd reply with today, but I was younger and more naive then.

I commented above that the initial questioning was fine.

> "or you don't know what your doing,"

That though was unprofessional, and should be addressed as such.

What would you reply today?

"Yeah, well I had sex with your wife!"

Or just offer over-inflated estimates since they won't accept something realistic anyway.

Not OP, but I will answer with the second option and ask him what I should do?

> A witty retort may be in order

I don't think this will score you a lot of points.

Depends. Are we talking self-esteem/dignity points, or good doggie/loyalty points?

I'd guess career-progression points, or even keep-getting-a-paycheck points at worst.

This low-stakes exchange won't affect much, it's about putting Mr. Incomp in his place. Remember it's a PM we're talking about, not "the Boss."