Definitions

1. adjective

being in the worst state of health; most unwell

“After catching a cold, he felt the sickliest he had ever been.”

2. superlative

most susceptible to illness; most likely to become ill

“She is the sickliest person in our class, always catching something.”

3. noun

the most unwell person or thing

“The sickliest patient in the ward required extra attention from the doctors.”