Sentence:  It seemed to them the ideal place for those confidential mid-morning potations and games of pool for which the Foreign Office had invented the phrase “elevenses”, after the hour of the morning when one feels one most needs a short swift drink, or a long “<WSD>unwinder</WSD>”, to use slang. 
Meaning of the word marked with <WSD> tags:  An alcoholic drink taken as a relaxant.