Robin Monroe: If you were half a pilot, we wouldn't be on this island!
Quinn Harris: I'm the best damn pilot you're ever going to meet!
Robin Monroe: Ha! I've flown with you twice and you've crashed half the time!
Robin Monroe: Pirates?! As in, 'Argh?!'.
Answer: Motu Nui is a real life island, the name means "large island" in Maori. Most likely that is the source of Lego's inspiration for the name. While it is certainly possible that whoever came up with Mata Nui first heard the name Motu Nui in this movie, it is unlikely. Motu Nui is a fairly well known island and also shares its name with a settlement in New Zealand. A fictionalized island called Motu Nui is also the home of Disney princess Moana, which obviously came out after Bionicle but speaks to how well known the name is.
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