Paolo mi amici,
As I was curious to know, I double-checked with an american colleague and here is what she wrote to me :
MONKEY BUSINESS can be used in several ways....it is a pretty innocent expression ;-)
Basically, it means something like 'up to no good' or 'fooling around'.....
For example:
Mom: 'I want no MONKEY BUSINESS while you should be doing your homework!'
It can also be a little naughty:
'When John and Mary sneak away by themselves, there's some MONKEY BUSINESS going on!'
OK???
Hope this helps.
Ciao,
Waïti.