noun
gentle outside, tough inside; iron hand in a velvet glove
A yojijukugo describing a person who appears gentle and mild-mannered but possesses inner strength, firmness, or resolve. Often used to praise someone's character.
彼は内剛外柔な人で、普段は穏やかだが芯が強い。
He is a person who is gentle on the outside but tough on the inside—usually calm but with a strong core.
Same meaning but with the character order reversed; 内剛外柔 is the more common form.
A yojijukugo composed of 内 (inside), 剛 (strong, firm), 外 (outside), and 柔 (gentle, soft). The exact origin is uncertain, but it follows a common four-character pattern contrasting inner and outer qualities.