I had one like this but it was so unbelievable. He was objectively 'stupid' but played into that - so he didn't have a clue who the Prime Minister was and bragged that he'd never read a book in his life. If he'd been open minded and curious I think he was intelligent enough to 'learn' but he thought he was too good for that.
He was also a raging misogynist. 'There isn't a single thing a woman can do that a man can't do better or faster.' Umm ok.
One day he became absolutely fixated on the idea that he could beat me at an IQ test. I had one done as a kid, was moved up a year in primary school but never told anyone. I've killed a fair few brain cells since then anyway. I was like, 'I mean, we can do one if you want, a proper one, but it costs money and the questions are hard. I've done one (didn't say my score).'
'Oh, well if you've done one it can't be that hard. I'll smash it. I probably wouldn't be that good at the literature questions as I don't read. But the geography questions, history questions, all of that, I'd smash it.'
'Um... mate. What do you think this is? A fucking pub quiz?'
I'm sure he's still utterly convinced he'd 'smash it' and beat me but alas, we never got the opportunity to find out because I dumped him. From similar experience though he'd have every excuse in the book. He was tired, distracted, I cheated...
Some of these male egos are so tedious. Presumably he gets a vibe that you're pretty smart - why does he feel the need to trample all over you and beat you?