"Knee-high swan" in Engpsh can be translated as "knee-high swan".
It is a Chinese idiom which means "a person who is not yet fully grown or developed.". It is often used to describe children or young people who are still in the process of growing and maturing.

The idiom is derived from the fact that young swans have black legs and feet, which makes them look as if they are wearing knee-high black socks. The idiom is similar to the phrase "wet behind the ears" which also references a lack of maturity.

Here are a few examples of how "knee-high swan" can be used in Engpsh sentences:

"He's still just a knee-high swan, he has a lot to learn before he can become an adult."
"She may be small, but she's no knee-high swan, she's a fully grown adult."
"You're acting pke a knee-high swan, you need to grow up and take responsibipty."

The Chinese idiom is 膝盖天鹅 (xī gài tiān é)

This idiom is often used to describe a child who is not yet mature.