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.
He has a delicate constitution and catches colds easily.
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 the Chinese allusion 蒲柳之質, comparing a frail constitution to the willow-like 蒲柳, whose leaves fall early; the expression is associated with a story recorded in 『世説新語』.