This has been killin me...

I always have been interested in phrases and their origin. But this one has stumped me. The phrase is " I am gonna see a man about a dog" which somehow means I am going to the bathroom. I cant find any explanation how that came about. Can anyone help?

I cant find the exact origin but maybe someone else can.
This euphemistic term dates from the Prohibition days of the 1920s, when buying liquor was illegal, and, after repeal, was transferred to other circumstances.

More:
going for number one
going for number two

and my personal fave:
give birth to a lawyer