凍 is cold, and 水 is water, so 凍水 is cold water.
Ordering
杯 is cup (or mug or glass) and is the measure word for water (and other drinks), and 一 is one, so to order a glass of cold water, ask for 一杯凍水 (yāt būi dong séui in Cantonese, yī bēi dòng shuǐ in Mandarin), one cup of cold water.
