See a man about a dog

To see a man about a dog, horse or duck is an idiom, especially British English, of apology for one's imminent departure or absence, generally to euphemistically conceal one's true purpose, such as going to use the toilet or going to buy an alcoholic drink. The original non-facetious meaning was probably to place or settle a bet on a race, perhaps accompanied by a wink. In the UK the phrase is generally used nowadays as a polite way of saying, "I am going out [or "have been out"], but don't ask where", often with the facetious implication that you are about to be, or have been, up to no good. In the Southern US, going to see a man about a dog signifies that one is going to urinate, while going to see a man about a horse signifies that one is going to defecate, and thus may be away a bit longer.

Historical usage
The earliest confirmed publication is the 1866 Dion Boucicault play Flying Scud, in which a character knowingly breezes past a difficult situation saying, "Excuse me Mr. Quail, I can't stop; I've got to see a man about a dog." Time magazine observed that the phrase was the play's "claim to fame". In Newcastle upon Tyne, Newcastle Brown Ale commonly gained the nickname of "Dog" from the frequent use of the phrase to describe going to the pub.

During Prohibition in the United States, the phrase was most commonly used in relation to the consumption or purchase of alcoholic beverages.