'untimbered' originates from Old English elements: the negative prefix 'un-' and the noun 'timber' (Old English 'timber'), where 'timber' meant 'building, structure' or 'wood for building' and 'un-' meant 'not.'

'timber' remained in English from Old English into Middle English as 'timber'; the participial/adjectival form 'timbered' (having timber) was used in Middle English, and the negated form 'untimbered' was formed by prefixing 'un-' to mean 'not timbered.'

Initially it referred specifically to something 'not provided with timber' (i.e., not built or not furnished with wooden supports); over time it was also used more broadly for land 'lacking trees' or simply 'not wooded.'