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firefly (n.)

also fire-fly, "An insect which has the faculty of becoming luminous" [Century Dictionary], 1650s, from fire (n.) + fly (n.). Ancient Greek had kysolampis "firefly, beetle with a tail that lights up at night," from kysos, kysthos "buttocks" + lampein "to shine."  

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Definitions of firefly from WordNet

firefly (n.)
tropical American click beetle having bright luminous spots;
Synonyms: fire beetle / Pyrophorus noctiluca
firefly (n.)
nocturnal beetle common in warm regions having luminescent abdominal organs;
Synonyms: lightning bug
From wordnet.princeton.edu