obstinate person; stubbornness; pigheadedness
Idiomatic expression for a person who is extremely stubborn or inflexible, or the quality of being so. Often used critically.
He's so stubborn that once he decides something, he never changes his mind.
It's impossible to persuade that obstinate person.
hard head (like a rock)
Literal meaning: a head as hard as a stone. Less common than the idiomatic sense, but can appear in physical descriptions or comparisons.
It hurt when I bumped it, but he has a hard head so he's fine.
My father is so stubborn that he never changes his mind once he's decided something.
It was pointless suggesting a new method to my stubborn boss.