Weighing less than is normal, healthy, or required.(adjective)
Insufficiency of weight.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "underweight" in a sentence
"Regular readers of this blog will have noted previous entries on the “paradoxical” reverse epidemiology of obesity and cardiovascular mortality, where risk is apparently higher in underweight compared to normal weight, overweight or even mildly obese individuals (for e.g. of previous blog entries on this click here, here or here)."
"Even more interestingly, it shows that being underweight is as much a danger to health as obesity, and that a few extra pounds in otherwise healthy adults seems to increase longevity."
"Although not approved by the FDA for this purpose, cyproheptadine has been used to stimulate appetite in underweight patients."