"Zek" is a Russian slang word for "prisoner" or "inmate", coming from заключённый (zakliuchyonnyi) which was usually abbreviated to "з/к" in Soviet paperwork, and pronounced as "зэка" ("zeh-KA"), usually in reference to a person who'd been imprisoned by the Soviet Union in the infamous gulag system for crimes against the state.
After the Soviet Union won the Cold War and spread its political ideology and system of government, the word "zek" became popular across the globe.
Generally, a country's zeks were political enemies, perceived or real, of the Communist regime; most countries used zeks as slave labor. While being a zek was not necessarily a death sentence in every country, those zeks who survived their prison sentences in harsh labor camps and were returned to society, lived out their lives as outcasts.
After visiting the sites of the Nazi death camps, NKVD officer Vladimir Bokov reflected on how the Naziconcentration camps were more wasteful than their Soviet counterparts: while the Nazi camps aimed to kill their inmates, the prisoners in the Soviet gulags were worked to death instead, gaining some labor for the Soviet regime.