in the period following the time when: He has written once since he left.
7.
continuously from or counting from the time when: He has been busy since he came.
8.
because; inasmuch as: Since you're already here, you might as well stay.
Origin: 1400–50; late Middle English syns, sinnes (adv.) thereupon, afterwards, Middle English sithenes (adv. and conjunction) afterwards, from (the specified time), because, equivalent to sithen after that, since (Old English siththan, orig. sīth thām after that; see sith) + -es-s1
mid-15c., synnes, from sithenes "since," from sithen (plus adverbial genitive -es), from O.E. siððan "then, later, after that," originally sið ðan "after that," from sið "after" + ðan, weakened form of ðam, dative of ðæt (see that). Modern
spelling replaced syns, synnes 16c. to indicate voiceless final -s- sound. O.E. sið is from PIE *se- "long, late" (cf. Ger. seit "since," Goth. seiþus "late," Skt. sayam "in the evening," L. serus "late").