underweight

underweight
un‧der‧weight /ˌʌndəˈweɪt◂ $ -ər-/ adjectiveweighing less than is expected or usual
   OPP  overweight:
    Women who smoke risk giving birth to underweight babies.
     

THESAURUS

■ person
   ▪ thin having little fat on your body: a tall, thin man
   ▪ slim thin in an attractive way: her slim figure | a slim woman in her fifties | Magazines are always full of advice about how to stay slim.
   ▪ slender written thin in an attractive and graceful way – used especially about parts of the body, and used especially about women: her long, slender legs | She is slender, with very fair hair.
   ▪ lean thin and looking healthy and fit: his lean body | He was lean and looked like a runner.
   ▪ skinny very thin in a way that is not attractive: a skinny teenager | Your arms are so skinny!
   ▪ slight written thin and delicate: a small, slight girl with big eyes
   ▪ scrawny /ˈskrɔːni $ ˈskrɒː-/ very thin, small, and weak-looking: a scrawny kid in blue jeans
   ▪ underweight below the usual weight for someone of your height, and therefore too thin: He had no appetite and remained underweight.
   ▪ gaunt /ɡɔːnt $ ɡɒːnt/ written very thin and pale, especially because of illness or continued worry: He looked gaunt and had not shaved for days.
   ▪ emaciated /ɪˈmeɪʃieɪtəd, ɪˈmeɪʃieɪtɪd, -si-/ written extremely thin and weak, because you are ill or not getting enough to eat: The tents were filled with emaciated refugees.
   ▪ skeletal written used about someone who is so thin that you can see the shape of their bones: The soldiers were shocked by the skeletal figures of the camp’s prisoners.
   ▪ anorexic used about someone who is extremely thin because they have a mental illness that makes them stop eating: Her daughter is anorexic. | anorexic teenagers