Origin and history of sith

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. Middle English had sitthen when "for what length of time?" (early 14c.).

"evening party," 1793, a French word in English, from French soirée, from soir "evening," Old French soir "evening, night" (10c.), from Latin sero (adv.) "late, at a late hour," from serum "late hour," neuter of serus "late," from PIE *se-ro-, suffixed form of root *se- (2) "long, late" (source also of Sanskrit sayam "in the evening," Lithuanian sietuva "deep place in a river," Old English sið "after," German seit "since," Gothic seiþus "late," Middle Irish sith, Middle Breton hir "long").

The French fem, suffix -ée, from Latin -ata, was joined to nouns in French to make nouns expressing the quantity contained in the original noun, and thus also relations of times (journée, matinée, année) or objects produced. Sometimes jocularly in 19c. swarry, a spelling representing English pronunciation.

adverb of time, "at that (specified past or future) time," Old English þanne, þænne, þonne, "in that case, under those circumstances," from Proto-Germanic *thana- (source also of Old Frisian thenne, Old Saxon thanna, Dutch dan, Old High German danne, German dann), from PIE demonstrative pronoun root *to- (see the).

Compare than, which originally was the same word. 

From late 13c. as "afterward, next in order." Also in Old English as a conjunction, "in that case, therefore." 

As an adjective, "being or existing as at that time" (then-husband) from 1580s, often then- and perhaps elliptical for then being. As a noun from early 14c., "that time" (as in by then).

Now and then "at various times" is attested from 1550s; earlier then and then (c. 1200). Then and there "at that time and place" is from mid-15c.