Apparently the English hate the French and that's where this term comes from. It's meant as an insult toward the French, insinuating that they are dirty, etc.  Here's one article that gives some great background on the immense dislike the English have for the French.  When an English person would do or say something crude the response would usually be along the lines of "pardon, are you French?" Here's another explanation for the history behind this term.  And one of the most well known uses of the phrase...

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