I'm not sure of the origin but it's basically a drink/snack of some sort, while I was waiting to go to Basic Flight Training I was at RMC getting shafted into being the duty officer every few weeks. In the evenings the RMC cadets used to get Kai and it consisted of juice, hot chocolate, cookies and other stuff like that, it all depended on the time of year as hot chocolate wasn't a very desirable thing during the warm weather.
Cheers