Throughout the debate Clinton referred to Trump as "Donald." Maybe the faux-friendly use of the first name through Trump off. If so, he fell for it, doubly so. It was not friendly, it was faux-friendly and that went right over his head. Hillary Clinton DETESTS Donald Trump. DETESTS him. Or maybe it was a double entendre by Clinton: faux-friendly and addressing him as an adult would a juvenile. Trump missed both. The Clinton campaign had identified Trump's intelligence as being one of three things that he was particularly insecure about.
How are you Donald?
CLINTON:...Finally, we tonight are on the stage together, Donald Trump and I. Donald, it's good to be with you.
His business acumen was a second of Trump's identified Big Three insecurities. Trump can accept no criticism. Here Clinton goes after both his business acumen and gets the intelligence insecurity in the bargain.
TRUMP: That’s called business, by the way.
And that was a dumb-ass response, Donald.