To be adamant about something is to have an opinion about it which you absolutely refuse to change, despite evidence to the contrary. It’s also, of course, a popstar from the 80s who I once dressed up as for a fancy-dress party (and yes, I did do the Prince Charming dance when I was there, despite being stone-cold sober due to driving, and also only knowing the crossed-arms move and nothing else). But the meaning of adamant as we know it only dates back to around the 1800s. Before that it was used as both a noun and an adjective for something that was really bloody hard – that’s hard like a diamond, not Jason Statham. In fact, ‘adamant’ was used as a synonym (i.e. another word for) a diamond.
If you’re a fan of the Marvel comic/film franchise you’ll probably have realised that this meaning is where ‘adamantium’ comes from – the fictional metal alloy that’s grafted onto Wolverine’s skeleton and claws and is virtually indestructible. ‘Adamant’ or ‘adamantine’ as an unbreakable substance also pops up in lots of classical literature – in Greek mythology, Cronus (Zeus’ dad) castrated his father Uranus using an adamantine sickle given to him by his mother Gaia. That must have made family gatherings quite awkward. And here it is in action in a particularly sexist bit of the novel ‘Romola’ by George Eliot (even though SHE WAS A WOMAN):
Trust not in your gold and silver, trust not in your high fortresses; for, though the walls were of iron, and the fortresses of adamant, the Most High shall put terror into your hearts and weakness into your councils, so that you shall be confounded and flee like women.
Oh, and it doesn’t have anything to do with the name Adam – that’s a Hebrew word meaning, basically, ‘man’.