This is a loanword -- the pronunciation (ga fē in Cantonese, kā fēi in Mandarin) sounds more or less like coffee.
The first character 咖 is similarly used in the loanword for curry, 咖喱 (ga lēi).
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 cup of coffee, ask for 一杯咖啡 (yāt būi ga fē in Cantonese, yī bēi kā fēi in Mandarin), one cup of water.
Coffee is not exactly a traditional dim sum drunk, but it's a common offering with Hong Kong breakfasts and lunches, particularly at Hong Kong-style cafes (cha chaan teng).