Useful idiot refers to people who are easily manipulated, brainwashed.
People who are a great way to propagandize a cause without fully comprehending the cause’s goals, and the entire ideology behind it.
A “Useful idiot” is manipulated and used by the cause’s leaders, a person who blindly follows any ideology and ignores the facts.
A popular belief is that the term was invented in the Soviet Russia, more specifically by Vladimir Lenin, to describe the people who are communist sympathizers in America.
They called them useful because they saw them as great tools to allocate communist propaganda to other countries, but as soon as the mission was completed those people were not “useful” anymore.
However, according to a 1987 article from New York Times, a journalist called William Safire dug deep into the case and found out that there isn’t an actual connection within Lenin and the term.
Similarly, the Oxford English Dictionary wrote: “The phrase does not seem to reflect any expression used within the Soviet Union”.
The term was originally used during the Cold War.
The earliest known appearance of the term was in a New York Times article in June 1948. The article was about Italian politics, where they mentioned a detail from the Italian newspaper called L’Umanitá, in which they used the term to refer to left-wing social democrats.
After that it appeared many times in the newspaper or in the news.
Nowadays, there are various sources on the internet with entries and definition about the phrase.