plain water

IPA^(key): /pləɪn ˈwɔːtə(ɹ)/

plain water (uncountable)

1. (Singapore) tap water, either hot or cold, served in restaurants.
   • 1971 June 17, The Straits Times, page 19:

     Politely I asked for a glass of plain water.

   • 1974 February 21, New Nation: Using Less Water, page 6:

     A visitor from Down Under is wondering why most Singapore restaurants and hotels make it a practice to serve all guests with plain water, even when it has not been asked for.

   • 1979 October 2, New Nation: Water: Expensive, page 8:

     Someone asked for a glass of plain water and was told by the waitress: “That’ll be 60 cents.”

   • 1987 June 13, The Straits Times: Install Water Cooler, page 20:

     Although there is a food and drink stall inside, there are joggers like me who prefer to drink plain water after sweating it out of the tracks.

Standard English speakers would simply ask for water as the word plain is redundant.