An Etymological Dictionary of the German Language/Annotated/Schleier
Schleier, masculine, ‘veil, pretence,’ from Middle High German sleier, earlier variants sloier, slogier, masculine, ‘kerchief, veil’ (the Middle High German term floier is curious); compare Dutch sluijer, Middle English sleir. Middle High German sloier, first recorded in the 13th century, is certainly a borrowed term; the assumption that it was introduced by the Crusaders from the East leads to no definite result. Perhaps it is connected with Old Irish sról, ‘silk.’