An Etymological Dictionary of the German Language/Annotated/zu

by Friedrich Kluge

zu, preposition, ‘to, in addition to, at, in order to’; adverb, ‘to, towards,’ from the equivalent Middle High German zuo (Middle German zû), Old High German zuo, zua, zô; compare the corresponding Old Saxon tô, Dutch toe, Old Frisian tô, Anglo-Saxon tô, English to; wanting in Old Icelandic and Gothic (for which Scandinavian til and Gothic du occur). It corresponds in non-Teutonic to Lithuanian da-, Old Irish do, as well as to Zend -da, Greek -δε, and Latin -do, which are used enclitically.