An Etymological Dictionary of the German Language/Annotated/dehnen

dehnen, verb, ‘to stretch, extend, lengthen,’ from Middle High German and Old High German denen, dennen, weak verb, ‘to stretch, draw, strain’; compare Gothic ufþanjan, ‘to extend’; Anglo-Saxon þenian, þennan, ‘to stretch.’ The Gothic þanjan is a derivative of a strong verb *þënan, like þakjan, ‘to cover,’ from a strong verb *þëkan (Latin tego); þanja and þëna are primitively cognate with Greek τείνω. The root ten is widely diffused in the Aryan group. Sanscrit root tan, ‘to strain, widen, extend (of time), endure’; tántu-s, masculine, ‘thread,’ tánti-s, feminine, ‘line, rope’; Greek τείνω, τάνυμαι, τάσις, τένων, ‘sinew,’ ταινία, ‘strip’; Old Slovenian teneto, tonoto, ‘cord,’ Latin tenus, ‘cord,’ Lithuanian tìnklas, ‘net.’ The idea of extension is shown also by the root ten (Latin teneo, tendo) in an old Aryan adjective; see dünn and Dohne. A figurative sense of the same root is seen in donnern; the evolution of meaning may be ‘extension—sound—noise.’