The term mukl, pronounced mook'l, originated in former Czechoslovakia during its Communist era. It means "political prisoner."
The word itself is an acronym, though always pronounced as a regular word. Its letters stand for Muž určen k likvidaci, Czech, roughly translated as man designated for liquidation.
This was not the official term, rather what the political prisoners called themselves. When most of them were released in 1968 (the year of the Socialism with a human face), the word mukl became known the general population and has been in general use ever since (including to describe the post-1968 political prisoners).
The very existence of a word created by the prisoners themselves from such an acronym is a tribute to human spirit. Some of these people spent 20+ years in prison with their only crime being that they did not agree with those in power. Many of them died in prison. Yet, they were able to use a certain sense of humor in creating the word to describe their own situation.
I have personally met many mukls in 1968, even one who had just been released days before!