What is the origin of the idiom "be off one's food" and when to use it?

"Be off one's food" is a commonly used idiom with no specific origin. It has developed over time as a way to describe a temporary loss of appetite or a decreased interest in eating. It is used to describe a situation where someone is experiencing a reduced desire or inability to eat. It suggests a lack of appetite, which can be due to various factors such as illness, stress, or emotional state.