The phrase "drink the Kool-Aid" is a derogatory term that refers to people who blindly follow someone or something without question, such as devotees of a particularly politician. Proper usage of the phrase makes reference to the beverage known as Kool-Aid, a reference to a mass death involving members of a cult called the People's Temple.
This cult was led by a man named Jim Jones, who established a commune for his cult in an area of South America that he called "Jonestown." Jones ended up either forcing or convincing about 900 people to drink a poisoned concoction that was flavored with packets of drink concentrate. Though it was not actually Kool-Aid, the name became associated with the mass poisoning and worked its way into the popular vernacular.