go to see a man about a dog

'First, I'm going to the nearest apothecary,' said William, 'and then I'm going to drop in at my lodgings for that key, and then . . . I'm going to see a man about a dog.'

British English uses "going to see my aunt", "going to see a man about a dog", "to piddle", "to splash (one's) boots", as well as "to have a slash", which originates from the Scottish term for a large splash of liquid.

The book's landscape and cast of characters are so selectively delineated that each of these forays has the mythic quality of a major undertaking, even on the journey about which "Uncle Al would say only that they were going to see a man about a dog."