He's crooked by nature, so it's no use saying anything to him.
That person has a firm character and is unshaken by any hardship.
気骨 also means backbone or fortitude, but is more formal and literary, often implying principled courage rather than innate nature.
Standard ateji spelling; the kanji 土性骨 are used phonetically.
Mixed kana-kanji variant; less common but seen in informal writing.
Formed from the emphatic prefix ど and 性骨（しょうぼね）, “innate character, spirit, or backbone.” 土 supplies the conventional spelling of the prefix.