San Fairy Ann is a bit WWI slang that emerged when British troops moved into France and Belgium; it is simply an English corruption of ça ne fait rien, a common French phrase meaning "it doesn't matter". It was a popular response indicating cynicism and resignation among the British soldiers, perhaps most akin to "whatever" in tone.

There are a number of variations in spelling (and capitalization), with 'san fairy ann' probably being the most common. The second most popular is probably San Ferry Anne, which was popularized when it was used as the title of a Paul McCartney & Wings song in the 1976.

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