word-forming element meaning "together with, jointly; alike; at the same time," also sometimes completive or intensive, from Greek syn (prep.) "with, together with, along with, in the company of," from PIE *ksun- "with" (source also of Russian so- "with, together," from Old Russian su(n)-). Assimilated to -l-, reduced to sy- before -s- and -z-, and altered to sym- before -b-, -m- and -p-. Since 1970s also with a sense of "synthetic."
Others are reading
<a href="https://www.etymonline.com/word/sy-">Etymology of sy- by etymonline</a>
Harper, D. (n.d.). Etymology of sy-. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/sy-