Underweight

[Sarah]

Underweight in Formula One. Describes a car that does not meet the minimum weight of the race. Results in disqualification.

[Donald Trump]

Below a weight considered normal or desirable.

[Jason Yan]

A term which is supposed to refer to a body mass index of less than [18.5] or simply being in a healthy weight range but is overused to categorize what seems like thin people in general.
Ridiculous really, just because someone weighs a couple pounds under the fixed ideal number and is clearly healthy they seem to be branded with a word that makes them sound [unhealthily] thin.
Meanwhile if someone weighs anything OVER the fixed ideal weight they are never called overweight until they actually reach the [BMI] 25 mark. In fact, women are complimented and praised for having great curves while if they weigh anything below that [optical] number they are considered [fit] for eating disorder diagnosis.