'well-behaved' originates from the combination of 'well' and 'behaved', where 'well' meant 'in a good or satisfactory way' and 'behaved' is the past participle of 'behave', meaning 'to act in a particular way'.

'behave' changed from the Middle English word 'behaven', which was derived from the Old French 'behaivre', and eventually became the modern English word 'behave'.

Initially, 'behave' meant 'to conduct oneself in a specified way', and this meaning has largely remained the same in modern usage.