noun
tough at heart; strong-willed inside
Describes a person who appears gentle or soft on the outside but is resolute and strong on the inside. Often used in contrast to 外柔 (がいじゅう, outwardly gentle).
彼は内剛の人で、見かけによらず芯が強い。
He is tough at heart, and despite his appearance, he has a strong core.
外柔内剛という言葉がある。
There is a saying: 'gentle on the outside, tough on the inside.'
Compound of 内 (inside) and 剛 (strong, firm). The exact historical derivation is uncertain, but it is typically encountered in the idiom 外柔内剛.