even-tempered; calm-minded; serene
Describes a person whose heart is calm and not easily disturbed; a steady, composed disposition.
He is always an even-tempered person.
It is difficult to stay calm-minded.
Compound of 心 (こころ, 'heart, mind') + の (possessive particle) + 平ら (たいら, 'flat, level, calm'). The phrase literally means 'flatness of heart', metaphorically extending to an even-tempered state.