having weak lungs; having a weak chest
Describes someone prone to respiratory problems or with a delicate constitution regarding the lungs. Often used as a set phrase.
He has weak lungs, so he's always coughing in winter.
I've had weak lungs since childhood and often got bronchitis.
A set phrase composed of 肺 (lung) + が (subject marker) + 弱い (weak). The exact origin is uncertain, but it follows a common pattern for describing bodily weaknesses.