E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Do as the cow o Forfar did, tak a stannin drink. A cow, in passing a door in Forfar, where a tub of ale had been placed to cool, drank the whole of it. The owner of the ale prosecuted the owner of the cow, but a learned baillie, in giving his decision, said, As the ale was drunk by the cow while standing at the door, it must be considered deoch an doruis (stirrup-cup), to make a charge for which would be to outrage Scotch hospitality. (Sir W. Scott: Waverley.)