"Monkey business" seems to be a widely used idiom. I looked it up on Wiktionary and I found two main possible meanings, one stressing the silliness of an activity:
(US, slang) Wasting time, or effort, on some foolish project. / An
activity that is considered silly, or stupid, or time-wasting.
the other one questioning its morality:
(idiomatic) An activity that may be considered illegal, questionable,
or a vice, but not felonious.
The Free Dictionary seems to favor the latter meaning.
Without any context that could help clarify the exact meaning, which of the two characterizations is usually understood?