Someone that follows orders without question and may carry out actions to help those that fed them the propaganda, such as disbelieves facts against whom they are biased toward, spreading inaccurate information in favor of their leader or unfavorably toward their opponent, and in general, act as a tool against their own self interest and the interests of those in whom they believe they know what's best for.
Sam believes he's fighting for freedom and liberation but he's bombing countries simply so that his government can take hold of the oil field and establish a central bank. Sam is a useful idiot for thinking freedom is the purpose, and those that support him as a troop are useful idiots for thinking their tax dollars don't kill innocent children in failed, illegal, drone strikes.
Rhonda is such a useful idiot because she thinks that Clinton matches her progressive ideals and chides those for supporting third party, yet, the two party system has been bombing countries for decades and making back door deals with large corporations. Most are tired of the corruption, yet Rhonda thinks that voting for those who are responsible for the corruption is somehow the best way to stop it. What a very useful idiot she is.
Cletus loves Trump because Trump says he won't send jobs out of the country, yet Trump's own products are manufactured out of the country. Cletus does not think Trump is a lying hypocrite because Cletus is a useful idiot.