na-adjective, noun
Describes a person or living thing that is physically frail, lacking strength, or prone to illness. Often used for children or people with a delicate constitution.
彼はひ弱な体つきをしている。
He has a weak physique.
ひ弱な子供はよく風邪をひく。
Delicate children often catch colds.
弱い is the general i-adjective for 'weak', while ひ弱 is a more specific na-adjective/noun emphasizing physical frailty or sickliness.
虚弱 is a more formal or medical term for constitutional weakness, whereas ひ弱 is more colloquial and often carries a nuance of delicateness.
The prefix ひ (sometimes written 非) can imply a negative or weak quality, combined with 弱 (weak). The exact historical derivation is uncertain, but the word has long been used to describe physical frailty.