Perhaps from שָׁעָהכְּ עַד (ád ksha'á) or הוּאכְּשֶׁ עַד (ád kshehú). Attested since Late Antiquity. The spelling with י may have arisen to disambiguate the vowel before ו so as not to be read as וֹ – as in analogous words like סתיו /סְתָו (stáv) and the possessive ending ־ָיו (-áv).