plural synechiae, "morbid union of parts, especially of the eye," 1842, medical Latin, from Greek synekheia "continuity," from synekhes "continuous," from syn "together" (see syn-) + ekhein "to hold" (from PIE root *segh- "to hold").
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."
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sahate "he masters, overcomes," sahah "power, victory;" Avestan hazah "power, victory;" Greek skhema "figure, appearance, the nature of a thing," related to skhein "to get," ekhein "to have, hold; be in a given state or condition;" Gothic sigis, Old High German sigu, Old Norse sigr, Old English sige "victory."
<a href="https://www.etymonline.com/word/synechia">Etymology of synechia by etymonline</a>
Harper, D. (n.d.). Etymology of synechia. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/synechia