un·der·weight 
/ˌʌndɚˈweɪt/ adjective
/ˌʌndɚˈweɪt/ adjective [more underweight; most underweight] : weighing less than the normal or expected amount : too light
▪ He is slightly underweight. ▪ She had become dangerously underweight. [=thin]
▪ He is slightly underweight. ▪ She had become dangerously underweight. [=thin]
