sin•syne(sin′sīn),USA pronunciationadv.[Scot. and North Eng.]
Scottish Termsfrom that time; since then.
1325–75; Scots and north, northern dialect, dialectal sin subsequent to, after (Middle English, variant of sithen, Old English siththansith) + syne; replacing Middle English (Scots) sensyne (sen, variant of sethen, variant of sithen; see since)