The compound adjective "well-behaved" is a transparent English formation. Its first element, "well," is an ancient Germanic adverb signifying goodness or propriety. The second element originates from the verb "behave," which combines the intensive prefix "be-" with the core concept of "having" or "holding" oneself. Thus, the literal construction "having held oneself well" evolved into the modern abstract meaning of conducting oneself in a socially acceptable and polite manner. It is a classic example of English's ability to create descriptive compounds from its native lexical stock.

Structure: well (in a good manner) + behaved (having acted) - well: From Old English wel, meaning "in a good manner, properly." It functions as an adverb modifying the participle. - behaved: From the verb behave, itself from Middle English behaven, from be- (thoroughly) + haven (to have, hold). The -ed suffix forms a past participle used adjectivally, meaning "having acted."