expression, i-adjective
frail; having a weak constitution; in poor health
Describes a person's overall physical weakness or susceptibility to illness. Often used to explain why someone gets sick easily or cannot do strenuous activities.
彼は体が弱くて、よく風邪をひく。
He has a weak constitution and often catches colds.
子供の頃は体が弱かったが、
I was frail as a child, but I'm healthy now.
病弱 is a more formal, written term for 'sickly' or 'weak constitution', while 体が弱い is a common everyday phrase.
虚弱 is a technical or medical term for 'frailty' or 'debility', often used in clinical contexts, whereas 体が弱い is conversational.
A straightforward phrase composed of 体 (body) + が (subject marker) + 弱い (weak). No complex etymology.