A body louse, especially the kind that infested soldiers' clothing during World War I; also used as an insult.
From Malay 'kutu' meaning louse, adopted into English during WWI when soldiers were regularly infested with these parasites in the trenches.
Cooties became so associated with WWI trench warfare that it became slang for any body louse, and kids later used 'cooties' as a joking insult—showing how a scientific term can become playground slang.
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