Historically and in British universities, refers to a pantry or refreshment room.
Definitions
A storeroom in a large house, monastery, or college where provisions, especially drinks, are kept and served. The steward unlocked the buttery to fetch a cask of ale for the evening meal.
A refreshment bar, especially in a British university, that sells snacks and drinks to students. After lectures, they met at the college buttery for tea and sandwiches.