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Old English fleon "take flight, fly from, avoid, escape" (contracted class II strong verb; past tense fleah, past participle flogen), from Proto-Germanic *thleukhanan (cf. Old High German fliohan, Old Norse flöja, Old Frisian flia, Dutch vlieden, German fliehen, Gothic þliuhan "to flee"), of unknown origin. Not found outside Germanic.
Weak past tense and past participle fled emerged Middle English, under influence of Scandinavian. Old English had a transitive form, geflieman "put to flight," which came in handy in the Anglo-Saxon Chronicle. Related: Fleeing.