Often used in reply to an awkward question, this phrase usually means that the responder is unwilling to reveal the true nature of his or her business.
It’s been a useful excuse for absenting oneself from company for about 150 years, though the real reason for slipping away has not always been the same. According to Eric Partridge, A Dictionary of Catch Phrases American and British, this phrase has had three meanings: Going to the toilet, going for a drink, or visiting one’s mistress.
However, the original meaning was probably to place or settle a bet on a racing dog.
My British grandfather use to excuse himself every morning by saying, “I have to see a man about a dog.” Much later, my grandmother explained that he was going to the bookies to bet on a horse race.
The phrase is old-fashioned and has a rural flavor. It evokes an image of life in a community where buying and selling dogs, trading dogs, betting on dogs, and consulting with men about dogs were routine errands, perhaps because dogs were essential for hunting or herding.
These days, the conventions of society have relaxed to the point where none of these reasons cause much consternation, so the phrase is usually only used in fun.
The earliest confirmed publication is the 1866 Dion Boucicault play Flying Scud in which a character knowingly breezes past a difficult situation by saying, “Excuse me, Mr. Quail, I can’t stop; I’ve got to see a man about a dog.” In a listing for a 1939 revival on the NBC Radio program America’s Lost Plays, Time magazine observed that the phrase was the play’s “claim to fame.”
My Canadian father, when we lived on a homestead in northern BC, often said, “I’ve got to see a man about a dog,” around ten in the evening, and then disappeared outside for a few moments. When he came back, he banked the fire and retired to bed. When I asked my mother what he meant, reasonably enough since our nearest neighbors were a mile away, she said bluntly, “He’s going for a pee.”