"sloping downward on both sides," 1833 (in Lyell), from -al (1) + Latinized form of stem of Greek synklinein "to incline, lean," from syn- "together" (see syn-) + klinein "to slope," from PIE root *klei- "to lean."
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."