"Going to see a man about a dog"
[A] Phrase used to conceal one's true destination, especially in response to an awkward question. Is commonly used in Britain when one is headed to the toilet.
"Are you and your wife coming out tonight?" "Uhhhh, I need to see a man about a dog."
"How come you're leaving the table?" "I need to see a man about a dog."
The Wikipedia entry says:
To see a man, to see a man about a dog, or to see a man about a horse is an English language colloquialism, usually used as a smiling apology for one's departure or absence - generally as a bland euphemism to conceal one's true purpose.
The phrase has several meanings but all refer to taking one's leave for some urgent purpose, especially to go to the bathroom or going to buy a drink. The original non-facetious meaning was probably to place or settle a bet on a race, thus dogs or horses.
During Prohibition in the United States, "to see a man about a dog" often meant to go meet one's bootlegger.
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." During a 1939 revival on the NBC Radio program America's Lost Plays, TIME magazine observed that the phrase is the play's "claim to fame".