See a man about a dog

If someone says, "I have to see a man about a dog" it means that he or she has to do something or other without being specific, but confidentiality is implied. It is mostly used in a surreptitious context e.g. going to the toilet, placing a bet, or sneaking off for a drink, the latter being most prevalent in the era of Prohibition. Its origin is thought to be American because its first citation is from the American, but Irish-born playwright, Dion Boucicault, in his play The Flying Scud or A Four-legged Fortune, which was published in 1866. There is also some evidence that the expression was in use in America and Ireland before this date. 'Dog' is often substituted for a 'horse' or 'duck', but 'dog' is still the favoured creature.