Garn is an (occasionally sarcastic) interjection used in Cockney slang and some other English dialects to express incredulity. It's a contraction of the phrase go on, but the locals rhoticized and slurred together the words until it became an entirely new term. According to the Oxford English Dictionary, it was first recorded in an 1886 dictionary of London slang in cant words, but it's not unusual for words like this to be used colloquially for many decades before they're written down. In other parts of England, garn can mean "yarn"; this is because, in Middle English, the word was spelled ȝarn, which used a yogh sound halfway between a y and a g, so it just developed both ways. That traces to Proto-Germanic garna and Proto-Indo-European ger, meaning "gut" or "intestine".
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