The state or quality of being underweight.

An underweight person.

To underestimate the weight of.

To give insufficient weight to (a consideration); to underestimate the importance of.

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.”

Being less invested in a particular area than market wisdom suggests.

“The fund is underweight in mining.”