see a man about a dog
Definition from Wiktionary, the free dictionary
- (idiomatic) Used as an excuse for leaving without giving the real reason (especially if the reason is to go to the toilet, or to have a drink)
Usage notes 
- The most common variation is to "see a man about a horse".
- Almost any noun can be substituted as a way of giving the hearer a hint about one's purpose in departing.
- The inversion to "see a dog about a man" eliminates any lingering uncertainty about whether the hearer is being put off.
- A shorter variant is to "see a man".