sinceyoudaoicibaDictYouDict[since 词源字典]
since: [15] Since is a contracted form of Middle English sithenes ‘since’. This in turn went back (with the addition of a final -s) to Old English siththan, a compound adverb and conjunction formed from sīth ‘after’ (a relative of German seit ‘since’) and thām ‘that’.
[since etymology, since origin, 英语词源]
since (adv.)youdaoicibaDictYouDict
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.