Describes a person or thing that is physically weak, fragile, or lacking in strength; often used for a slender, delicate build or constitution.
She has a delicate physique.
That flower is frail and cannot withstand strong winds.
The kanji 繊 (fine, slender) and 弱 (weak) combine to express delicacy and frailty. The reading せんじゃく is the on'yomi of both characters.