'behaviour' originates from Middle English and early Modern English, ultimately formed from the verb 'behave' (be- + have).

'behaviour' developed from the verb 'behave' (formed from Old English elements 'be-' and 'habban'/'have') and was shaped into the noun form in Middle English and Early Modern English (compare Medieval/Old French influences on noun endings).

Initially it referred broadly to the manner of conducting oneself or the way something was managed; over time it narrowed to mean the observable actions or conduct of people, animals, or systems.