Firstly! You rock and thank you for spreading this amazing game to so many people! Secondly, I am going to be playing a monk a “from rags to riches” concept based on Sun Wu Kong, and am needing an official base guideline on nunchaku! Please & thank you!I’d treat it as a flail
— Mike Mearls (@mikemearls) March 26, 2018
In the Monk section of the PHB it expressly states that a nunchaku is a club.
This is also important, since a flail is not a valid monk weapon and therefore would not be able to be used by the character (unless they were to go Kensei).