from The American Heritage® Dictionary of the English Language, 4th Edition
- n. An inmate of a Soviet labor camp.
Russian, from abbr. for zaklyuchënnyĭ, prisoner, from past passive participle of zaklyuchat', to imprison, from klyuch, key, from Old Church Slavonic ključĭ.(American Heritage® Dictionary of the English Language, Fourth Edition)
From Russian зэк (zèk), probably representing a pronunciation of з/к (z/k), Soviet abbreviation of заключённый (zaključënnyj, "prisoner"). (Wiktionary)