The word Cha is tea and Yen is cold. The word Cha Yen therefore means Iced Tea, as well as another meaning that is being aloof and cold. Leaving that aside, the important word that you should really remember before ordering Cha Yen from a local cart or shop is the word "Mai Waan" meaning not sweet. Without this instruction (or even with one), you can expect loads of sugar plus sweeten condensed milk in your drink.. I mean loads!
You've been warned :)