Brewer's: Stirrup Cup
A “parting cup,” given in the Highlands to guests on leaving when their feet are in the stirrups. In the north of the Highlands called “cup at the door.” (See Coffee.)
Lord Marmion's bugles blew to horse; Then came the stirrup-cup in course; Between the baron and his host No point of courtesy was lost.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894