What is the origin of the idiom "weak in the knees" and when to use it?

The exact origin of the idiom "weak in the knees" is unclear, as it has been in use for a considerable period of time. However, it is thought to stem from the physical sensation that some people experience when they are overcome with strong emotions or excitement. It is commonly employed when someone experiences a strong, overwhelming, or captivating reaction to a person, event, or situation.