An Etymological Dictionary of the German Language/Annotated/welch
welch, pronoun, ‘which, what,’ from Middle High German wëlch, wëlich, interrogative pronoun, Old High German wë-, wie-lī̆h (hh and h), interrogative pronoun, ‘who, which’; corresponding to Gothic hwileiks (hwêleiks), Old Saxon hwilik, Dutch welk, Anglo-Saxon hwylč, English which. A common Teutonic derivative, from the pronominal stem hwe- (see wer) and the suffix lîko-, ‘constituted’ (see gleich and solch); hence welch, literally ‘as constituted.’