'thoroughly-researched' originates from the combination of 'thoroughly' and 'researched', where 'thoroughly' means 'in a thorough manner' and 'researched' is the past participle of 'research', meaning 'to investigate systematically'.

'thoroughly' changed from the Old English word 'thurh' meaning 'through', and 'researched' evolved from the Old French 'recercher', meaning 'to seek out'.

Initially, 'thoroughly' meant 'through' or 'completely', and 'researched' meant 'to seek out'. Over time, it evolved to mean 'extensively investigated'.