Definition: [adjective] below a weight considered normal or desirable.
Example: he was thirty pounds underweight
Definition: [adjective] having less investment in a particular area than is normal or desirable.
Example: the company is still underweight in Japan
Definition: [verb] apply too little weight to.
Example: we feared the hot-air balloon had been underweighted
Definition: [noun] insufficient weight.
Example: thinness and underweight may be a physical quality that you and I share