underweight [ˌʌndərˈweɪt]

例句:

• Nearly a third of the children were severely underweight.
  近1/3的孩子体重严重偏轻。
  
• The underweight child has begun to fill out after regular exercises.
  这个偏瘦的孩子在进行了一段有规律的锻炼后开始长壮了。
  来自《权威词典》
• The coal is six pounds underweight / underweight by six pounds.
  这些煤的重量少了六磅.
  
• My son was underweight when he was born.
  来自WSJ
• The survey says many children in India are born underweight.
  来自VOA
• Otherwise we would not be over- or- underweight by the billion.
  来自SA