Use monkey business to describe what your sneaky, mischievous little brother gets up to when no one's watching him. Climbing on top of the refrigerator or filling the sugar bowl with salt could both be described as monkey business.
Monkey business is one of those animal idioms that make a lot of sense — just picture a rascally monkey stealing the hat off your head and running away. The phrase first came into use around the start of the twentieth century, possibly from the earlier term monkeyshines, which had the same meaning. It's thought they were influenced by the increasing popularity of zoos in the United States in the late 1800's.