Great question — answering as the UphillTech team:
http://www.nomachetejuggling.com/2011/11/11/the-star-wars-saga-suggested-viewing-order/ was where the order was introduced. The name of the order is from the author Rod Hilton's blog name, "Absolutely No Machete Juggling" ( blog name origin is a cartoon about human nature, NOT about actual blades ) I've dubbed this "Machete Order" on the off chance it catches on because I'm a vain asshole. (UPDATE: It did!)
Full answer by DVK: https://movies.stackexchange.com/a/65253 (CC BY-SA 3.0).
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