The verb is from Middle English endeveren (“to make an effort”); the noun is from Middle English endevour, from the verb. Endeveren is from (putten) in dever (“(to put oneself) in duty”), from in + dever (“duty”), partially translating Middle French (se mettre) en devoir (de faire) (“(to make it) one's duty (to do), to endeavour (to do)”) (from Old French devoir, deveir (“duty”)).