noun
obstinate person; stubbornness; pigheadedness
Idiomatic expression for a person who is extremely stubborn or inflexible, or the quality of being so. Often used critically.
彼は石頭だから、一度決めたら絶対に意見を変えない。
He's so stubborn that once he decides something, he never changes his mind.
あの石頭を説得するのは無理だ。
It's impossible to persuade that obstinate person.
noun
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.
ぶつけたら痛かったけど、彼は石頭だから大丈夫だ。
It hurt when I bumped it, but he has a hard head so he's fine.