Physical frailty or lack of strength; a weak constitution or delicate health.
祖母は年を取ってとても虚弱になった。
My grandmother has grown very frail with age.
彼は虚弱体質で、少しの寒さでも風邪をひく。
He has a weak constitution and catches colds even in slight cold weather.
虚弱さを補うために、医師の指導で運動を始めた。
To make up for his frailty, he started exercising under his doctor's guidance.