cup sake; single-serve sake
Refers to sake packaged in a small glass cup or can, often sold in convenience stores and vending machines. A casual, affordable way to drink sake.
コンビニでカップ酒を買って、公園で飲んだ。
I bought a cup of sake at the convenience store and drank it in the park.
Compound of カップ (cup) and 酒 (sake), literally 'cup sake'.