See a man about a dog

Common card · No. 223899

To see a man about a dog or horse 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 United Kingdom, its modern usage is a polite way of a person saying that they are going away, or have returned from somewhere, but do not want to be asked where, often with the facetious implication that they are up to no good. In the Southern United States, 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.

Read on Wikipedia