In the Malay language, "air" means "water"; "kathira," or "katira," offers a sticker problem in translation. The proprietor, and a couple of passers-by, could offer no insight except that it's a common fast-breaking beverage during Ramadan; as with eggnog, many drink it, but few can elaborate on the etymology. Possibly "kathira" is a loanword from Urdu, denoting a gum derived from sap and used as a thickener.
Of several available flavors, I eventually settled on pale green pandan (S$2.00, about US$1.50 at the time) over golden durian.
Air kathira stall
At a Ramadan bazaar on Geylang Road near the Joo Chiat Complex, Singapore
(From a summer 2010 visit)