To see a man about a dog is an expression I grew up with, meaning I am going to go urninate. In most late 19th century and early to mid 20th century "proper" American and British families it was not considered proper to just say "I gotta go pee" and you were expected to use a euphamism such as the above expression. I don't know that this was the original meaning of the expression, but I first heard it used that way nearly fifty years ago.