From Longman Dictionary of Contemporary Englishdumb waiterˌdumb ˈwaiter noun [countable] DFUa small lift used to move food, plates etc from one level in a restaurant, hotel etc to another
Examples from the Corpus
dumb waiter• The kitchen is equipped along old-fashioned lines, and meals reach the dining room via a dumb waiter.• Where design permits, the old-fashioned serving hatch can come in handy: it's the dumb waiter of the less-palatial home. 