noun
delicate health; fragile constitution
Literary set phrase meaning a weak or easily damaged physical condition, like a willow in the wind. Often used self-deprecatingly or to describe someone's poor health.
彼は蒲柳の質で、すぐに風邪をひいてしまう。
He has a delicate constitution and catches colds easily.
私も蒲柳の質ゆえ、無理はできません。
I too am of frail health, so I cannot overdo it.
虚弱体質 is a medical/formal term for 'constitutional weakness'; 蒲柳の質 is a poetic, classical way to express the same idea.
From classical Chinese literature, where 蒲柳 (willow) was used as a metaphor for something weak and easily swayed, combined with 質 (nature/constitution). The exact historical derivation is uncertain, but the phrase has been used in Japanese for centuries to describe frail health.