sine n.

early 15c., synnes, sinnes, a contraction of sithenes "since," from sithen (with 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).

It was used from late 14c. as a conjunction, "from the time when, during the time after." As a preposition, "ever, from the time of," from 1510s; "from the time when," hence "as a consequence of the fact that." The modern spelling replaced syns, synnes 16c. to indicate a voiceless final -s- sound. The meaning "before now, ago," with some specifying word or phrase (long since, etc.) is from late 15c. Since when?, often expressing incredulity, is attested from 1907.

in Burns' poem "Auld Lang Syne" (1788), the usual Scottish form of obsolete sine (adv., conj.) "after that, afterward; before now, ago," c. 1300, an adverb from Middle English sin, a contraction of sithen "afterward, from now on," for which see since. Syne is the same word as since, but without the adverbial genitive inflection.