noun-modifying (irregular)
Describes a person with strong, unwavering spirit; often used for someone who stays calm under pressure. Functions as a prenominal modifier.
彼は根性のすわった男だ。
He is a fearless man.
根性のすわった選手だけが、あの場面で冷静でいられる。
Only a gutsy player can stay calm in that situation.