'timbered' originates from Old English and other Germanic sources, specifically the word 'timber' (Old English 'timber' / Old Norse 'timbr'), where the root meant 'building, structure; wood for building'.

'timbered' developed from the Old English noun/verb 'timber' (meaning building or wood for building) into Middle English forms (e.g. 'timber', 'timbrid') and later the modern adjective and past-tense/past-participle forms 'timbered'.

Initially, the root referred broadly to 'building' or 'wood for building'; over time the sense narrowed/shifted to things 'made of or provided with timber' and to the verbal sense 'to provide or fell for timber', reflected in 'timbered'.