The compound adjective 'well-researched' is a modern English formation, combining the intensifying adverb 'well' with the past participle of the verb 'research'. The verb 'research' itself entered English in the late 16th century from Middle French, where it literally meant 'to search again', implying a diligent and repeated seeking of information. The suffix '-ed' transforms this action into a descriptive state. Therefore, the word's logic is transparent: something described as 'well-researched' has undergone the process of investigation ('research') in a thorough and commendable manner ('well'), resulting in a reliable and informed state.

Structure: well (thoroughly) + research (investigate) + ed (adjective suffix) - well: Old English origin, from the adverb 'wel' meaning 'in a good way, thoroughly'. In this compound, it functions as an intensifying adverb. - research: Derived from the French 'rechercher', from 're-' (again) + 'chercher' (to search). It functions as the core verb root meaning 'to investigate systematically'. - ed: Old English origin, a past participle suffix used here to form a participial adjective indicating a completed action or state.