Definition & Meaning of "(as|) weak as a kitten"

What is the origin of the idiom "weak as a kitten" and when to use it?

The idiom "weak as a kitten" originated from the observation that kittens are small and lack strength compared to fully grown cats. It has been in use since at least the 19th century to describe someone who is extremely weak or lacking in strength. It is used to describe someone who is very physically weak or lacking energy. It emphasizes the person's lack of strength by comparing them to a small, fragile kitten.