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Commonly used today as a description for unsolicited messages, the word "spam" became associated with repetition after the infamous 1970s Monty Python skit which is set in a café with Vikings and features a cross-dressing hostess who comically repeats SPAM (spiced ham) as a menu item.
It's sometimes said that spam emails were associated with being "fake" messages in the same way that SPAM is called fake meat, but there is little to substantiate this.
The exact origin point of spam being used as computer lingo is unknown, although early examples date back to chat systems and multi-user dungeons of the 80s. The term caught on after a husband and wife team of lawyers bulk posted ads for immigration law services on Usenet groups in the early 90s.