congress of the Polish republic, 1893 (with an isolated reference from 1690s), from Polish sejm "assembly," from *syn-imu, literally "a taking together," from *syn- "together" (see syn-) + PIE root *em- "to take."
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 yamati "holds, subdues;" Latin emere "buy," originally "take," sumere "to take, obtain, buy;" Old Church Slavonic imo "to take;" Lithuanian imu, imti "to take."
For the sense shift from "take" to "buy" in the Latin verbs, compare Old English sellan "to give," source of Modern English sell "to give in exchange for money;" Hebrew laqah "he bought," originally "he took;" and colloquial English I'll take it for "I'll buy it."
<a href="https://www.etymonline.com/word/Sejm">Etymology of Sejm by etymonline</a>
Harper, D. (n.d.). Etymology of Sejm. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/Sejm