What sickness was 瘦?
「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」.