The phrase "you're welcome" used as a courtesy in English comes from the Old English "you wear wellingtons", and was used as an insult by the Anglo-Saxons against their Norman conquerors. The phrase implies that the person wears wish wellingtons, that is, that they are William, and thus that they are William Pitt, rhyming slang for excrement. All of this despite being developed centuries before either William Pitt or William's Wish Wellingtons were known.