Since
late Middle English: contraction of obsolete sithence, or from dialect sin (both from dialect sithen ‘thereupon, afterwards, ever since’).
wiktionary
From Middle English syns, sinnes, contraction of earlier sithens, sithence, from sithen(“after, since”) ( + -s, adverbial genitive suffix), from Old English sīþþan, from the phrase sīþ þǣm(“after/since that (time)”), from sīþ(“since, after”) + þǣm dative singular of þæt. Cognate with Dutch sinds(“since”), German seit(“since”), Danish siden(“since”), Scots syne(“since”).
etymonline
since (adv.)
early 15c., synnes, from sithenes "since," from sithen (plus adverbial genitive -es), from Old English siððan "afterward, from now on, hereafter, further, later, as soon as, after that," originally sið ðan "after that," from sið "after" (see sith) + ðan, weakened form of ðam, dative of ðæt (see that).
As a conjunction from late 14c.; as a preposition from 1510s; "from the time when," hence "as a consequence of the fact that." Modern spelling replaced syns, synnes 16c. to indicate voiceless final -s- sound. Since when? often expressing incredulity, is from 1907.
