To obtain extra luck by adding an extra item or 'gibbon' to what you already have.
This phrase comes from Southern China, where the nobel gibbons where seen as "
the gentlemen of
the forest". These gibbons were collected by wealthy lords and it was common for a lord to add an extra gibbon to his collection to bring enhanced good fortune.
In modern times it has become a phrase used simply to justify overindulging. This phrase is now used less in China and more in Malaysia, Thailand and Sumatra where gibbons still thrive
in the wild.