This expression refers to someone who is soft and gentle on the outside, but inwardly tough and resilient. Similar to the English expression “steel in a velvet glove”.
This is a special kind of Japaneses idiom known as a 四字熟語 (よじじゅくご、yo-ji-juku-go), or 4-character idiom. As the name suggests, these are idiomatic phrases made of 4 kanji characters.
They often contain a small life lesson or moral message, similar to English sayings such as “you reap what you sow” and “the early bird gets the worm”.
Calligraphy by Vicky 