What's the usual meaning of "monkey business"?

"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?

Solution 1:

I think the two meanings are so close it's hardly practical to try and split usages into one or the other. Lots of things can be called "monkey business" - in if I catch him up to any monkey business with my sister, I'll bash him, it simply means making sexual advances.

Other words with the same broad spread of meanings include shenanigans, mischief, skullduggery, roguery, nonsense, funny business/stuff, antics, horseplay. It's a matter of context whether the activity thus described is immoral, or simply frivolous, but it's invariably somewhat disapproved of.

Solution 2:

I would say the former is usually understood. I can see it extending to the latter, but I always think of monkey business as something more benign.