What is the origin of the idiom "keen as mustard" and when to use it?

The phrase "keen as mustard" originated in the early 19th century and refers to someone's intense enthusiasm or eagerness. It draws on the strong and pungent flavor associated with mustard to symbolize a person's passionate nature. The phrase has since become a commonly used idiom to describe someone who is highly motivated or excited about something. This idiom is typically employed in informal conversations or informal writing to convey a sense of strong excitement or motivation.