An idiomatic expression that denotes fooling around or acting in a silly manner. Example: “when the salesman was fooling around in the outlet, the manager warned him not to do any more monkey business, or he will otherwise have to face the repercussions of silly behavior.”

It also means doing something that is fraud or dishonest. Example: “the firm is expected to do no more monkey business in the aftermath of the sector-wide scandal that hit all manipulators, and the measures that have been imposed by regulators.”