A combe downer is a "less-than-full" mug or cup of your chosen beverage. It originates from 1970s Bath, UK. People from Combe Down were said to be stingy and so therefore, a "combe downer" is a stingy serving, usually a drink.
Person 1: *makes tea, half fills your mug, hands it to you*
Person 2: "well, that's a bit of a combe-downer..."