What sickness was 瘦? “Skinny” (瘦) is clearly written with a sickness radical (病字旁): 疒.

Outlier, also, writes the following in its definition:


  FORM
  瘦 shòu is composed of 叟 sǒu and 疒 “to be sick, not well,” pointing to the original meaning “to have little muscle or fat; to be skinny.” 叟 gives the sound.


What sickness was 瘦 actually referring to though? Or is it simply that all these people vying to loose weight just can't wait to be “ill”?
「瘦」is comprised of semantic「疒」(sickness) and phonetic「叟」, indicating the meaning underdeveloped muscles and fat; emaciated (speaking about the body).



The skinny meaning is not how we view the meaning skinny in modern terms.


Today, especially in countries which are not struck by famine or poverty, skinny is viewed as a better state than obese. To become skinny/thin is to change from an unhealthy obese weight.
In the olden days, skinny is a synonym for malnutritioned/pallid/emaciated, hence the use of「疒」.