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.