McCarthyism means 'political persecution without process of law'. It's named after US senator Joe McCarthy, an anti communist crusader during the early Cold War. The House Committee on Un-American Activities was created to weed out communists and sympathizers, but became a sort of witch hunt, with government organizations and industries conducting their own investigations. These actions reached a pitch of hysteria, in which even suspicions were enough to get people fired and destroy careers. Many people were blacklisted for years in Hollywood, purely on allegations. The egregious FBI director J Edgar Hoover played a major part in spreading the anti communist frenzy, and the resulting loss of thousands of jobs.
Examples of McCarthyism:
The Hollywood blacklist
A 1947 propaganda comic book published by the catechetical guild educational society warning of the dangers of a communist takeover.