http://www.wordorigins.org/
Great place to find out where strange words or phrases come from.
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Tongue in Cheek:
The commonly told tale about this phrase is that it derives from the acting practice of thrusting one's tongue into your cheek to keep from laughing at an inappropriate moment. There is no evidence to support this story or the idea that the phrase comes from the theater.
The phrase dates to 1748 when it was fashionable to signal contempt for someone by making a bulge in your cheek with your tongue. By 1842 the phrase had acquired its modern, ironic sense.
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Chris Woods