Underweight (adjective)

1. Having less weight than is considered healthy or normal, often measured by body mass index.
2. Lacking the necessary or adequate amount of weight, bulk, or substance.

Origin:

From under- + weight, ultimately from old english weġtan, "to weigh".

Examples:

1. The patient was underweight, having a low body mass index and poor nutritional status.
2. The horse was underweight, lacking muscle mass and body condition.
3. The package was underweight, not containing the advertised amount of product.
4. The building was underweight, lacking sufficient structural support and stability.
5. The argument was underweight, lacking evidence and logic to support the conclusion.