go see a man about a dog. A euphemism for relieving one's bladder, used primarily by men.
This is one my father used. He was a man of many colorful phrases and this was hardly the only one that he used to indicate that he had to relieve himself. Neither is it the most cute, nor to any extent the least acceptable in polite company. It seemed to refer exclusively to urination, though frankly that would be more information than I would care to know, now that I think about it. The origin appears to be less specific. The American Heritage® Dictionary of Idioms by Christine Ammer dubs it an “excuse oneself without giving the real reason for leaving” and names its origin this way: “This euphemistic term dates from the Prohibition days of the 1920s, when buying liquor was illegal.” Well, my dad might even have known that, but he never told me. The Phrase Finder offers an even broader meaning: “Warning that you are about to do something in a secretive manner and that you don't want to talk about it until later.”
What does dog mean in this context? As far as I can tell, the phrase is used to discourage further inquiry. It refers to commonplaces, a man and a dog, and is of no great import. Perhaps it was simply supposed to be a polite way of saying that he didn't want to talk about it, though the coyness with which he delivered the phrase suggests otherwise.
“Go kill a dog” is sometimes used to indicate going to defecate. Here the reference is much more direct, alluding to the smells associated with the act.
1. Ammer, Christine. 1997. The American Heritage Dictionary of Idioms. Boston: Houghton Mifflin.
2. Martin, Gary J., ed. 2001. The Phrase Finder. Sheffield Hallam University. Accessed Aug 26 2001 from http:// www.shu.ac.uk/web-admin/phrases/.
3. Crawley, LaVera M. 2001. Go Kill a Dog. to A. MacLeod. conversation, Sep 30.