What is the origin of the idiom "monkey business" and when to use it?

The idiom "monkey business" originated in the mid-19th century, deriving from the association of monkeys with mischievous and playful behavior. Monkeys are known for their agility, curiosity, and tendency to engage in antics, and the term "monkey business" was initially used to describe frivolous or deceitful activities. Over time, the expression evolved to describe any behavior perceived as unethical, dishonest, or mischievous, often referring to actions that deviate from serious or legitimate conduct.