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From Japanese 馬鹿野郎 (ばかやろう, bakayarō, "idiot"). Popularised by TV series on the Second Sino-Japanese War in Mainland China, where the use of this interjection by Japanese officers usually symbolises the brutality of the Japanese army. Often pronounced /ˈbagəjalu/. See also 日本軍隊中國語 ("Chinese language spoken by the Japanese army") on Wikipedia.