What 'monkey business' does really mean?

Have you not seen the climbing monkey?

When he is among the plane trees, such as

Rattans, oaks and camphor trees, he grasps

And twists their branches into a screen,

Where he reigns quite at his ease, so that

Not even James Bond could spy him out.

When, however, he finds himself

Among the prickly mulberry and date trees

And other thorns, he goes cautiously,

Casts sidelong glances and takes

Every trembling movement with apprehension --

It is not that his sinews and bones are straitened

And have lost their suppleness, but

The situation starts to be unsuitable for him

And he can display his agility no more.

This is what can be considered

As the essential meaning of the phrase,

Which leads us to another wisdom --

Discretion is the better part of valour.