Plural of banzai; the Japanese exclamation 'banzai' (literally 'ten thousand years'), often shouted as a cheer or cry of celebration or patriotism.
From Japanese '万歳' (banzai), combining '万' (ban, ten thousand) and '歳' (sai, year), originally a wish for long life and prosperity.
The banzai cheer became so associated with Japanese military culture in World War II that Western soldiers initially thought it was a battle cry, when it simply meant 'long live!'
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