What does underweight mean?
Underweight means below the normal, expected, or required weight. It is commonly used in medicine and nutrition to describe a person or animal whose weight is lower than considered healthy for their age, height, or circumstances. The related noun, “an underweight,” denotes such a person or animal. In formal and technical writing, underweight can also function as a verb meaning to give insufficient weight or importance to a person, factor, argument, or possibility: for example, a model may underweight the risks of a decision. The word is used in both British and American English. Although usually neutral and precise in clinical or analytical settings, it may sound blunt when applied directly to a person's appearance, making “below a healthy weight” a more considerate alternative. Underweight is also easily confused with lightweight, which often indicates a category of relatively low weight rather than a weight that is abnormally or unhealthily low.
Below the normal, expected, healthy, or required weight.
"She was slightly underweight for her height and age."
Common in medical, nutritional, agricultural, and sporting contexts. In personal or social contexts, “below a healthy weight” may sound more tactful.
A person or animal whose weight is below the usual or healthy range for their age, height, or circumstances.
"The shelter provides special diets for underweight cats."
Usually used in medical, nutritional, veterinary, or animal-care contexts.
To give insufficient weight or importance to someone or something; also, less commonly, to be below a specified or standard weight.
"The study may underweight the influence of weather conditions."
The figurative sense is formal and especially common in analytical, academic, and technical writing.
The plural is used when referring to multiple people or animals whose weight is below the usual or healthy range.
"The clinic evaluated several underweights referred by local schools."
Underweight has two lives: it can describe a body below a healthy range and a model that gives something too little influence.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does underweight come from?
Underweight is an English compound formed from under and weight. Under derives from Old English under, meaning “beneath” or “below,” while weight comes from Old English wæġt; the compound is attested from the 17th century. Words such as weigh, wight, and outweigh share the Germanic ancestry associated with weight, although they developed different meanings and grammatical forms.
Related word forms
How underweight is actually used
In medical contexts, the term is standard but can sound blunt when describing a person; phrases such as “below a healthy weight” may be more tactful. The verb is formal or technical and is often used figuratively, especially in research, analysis, and debate.
Easily confused with underweight
Underweight specifically means below an expected or healthy weight, whereas lightweight often denotes a low-weight category or a person of slight build without implying that the weight is abnormally low.