underweight
adjective
°Of an inappropriately or unusually low weight.
"He's so underweight he's had to buy smaller clothes."
°Not too heavy for an intended purpose.
"The suitcase is just slightly underweight; I'll let it on the plane."
noun
°The state or quality of being underweight.
verb
°To underestimate the weight of.
°To give insufficient weight to (a consideration); to underestimate the importance of.