The Origin of Thank-you-ma'am?

Thank-you-ma'am means "a bump or depression in a road". But does anyone here know why a bump or depression of a road could be called "thank-you-ma'am"? Here I try to guess it with my imagination (no doubt the knowledge of my etymology is poor now, so show your rich imagination please).
Once upon a time, there is a gentleman who is sitting in a carriage with several kissable ladies to take a trip from London to country. He really wants to kiss one of them, but finds no chance to do so. When the carriage has been away the smooth streets of London, and driving on the rough surface of a road of country, which is full of bumps and depressions, the pretty ladies become involuntarily wavering and sometimes bump gently into the gentleman, who, of course, enjoys the gentle bumps with great joy. No wonder, the gentleman thus calls a bump/depression in a road "thank-you-ma'am" since then.