Meaning

noun an alliance made up of states that had been Soviet Socialist Republics in the Soviet Union prior to its dissolution in Dec 1991

noun a unit of radioactivity equal to the amount of a radioactive isotope that decays at the rate of 37,000,000,000 disintegrations per second

originLikely originates from the false etymology asserting that cis is an initialism standing for comfortable in skin.

Etymology from Wiktionary, CC BY-SA 3.0