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.