drink the Kool-Aid: follow a leader blindly or commit unquestioningly to a set of beliefs
Drinking Kool-Aid, a favorite beverage since Edwin Perkins invented it in 1927, took on a darker significance in 1978. In that year, over 900 members of the People's Temple committed suicide by drinking poisoned Flavor-Aid, a beverage similar to Kool-Aid.
Jim Jones founded the People's Temple in 1955 Indianapolis. In 1965, Jones moved with his family and church members to California, where the group swelled to about 1,000. Partly to get away from disaffected former members and members' families, who accused the organization of cultish practices, Jones and his followers moved to Guyana in 1977. They established a settlement called Jonestown. The move did not stop the complaints against them back in the United States.
In November 1978 Democratic Congressman Leo Ryan flew to Jonestown with a team of investigators. After their visit to the town, as they arrived back at the airport, several Jones loyalists drove onto the tarmac and opened fire. They killed Ryan and five others. Meanwhile, events had reached a crisis in Jonestown. Jones called together church members for a suicide rehearsal, which he had done before. However, this time it was not a rehearsal. Several people began passing out cups of Flavor-Aid laced with cyanide and Valium. Although a number of members hid or fled into the jungle when they realized that the drink was poisoned, 909 people died, including over 200 children. Jones himself was found dead of a bullet wound. Whether it was self-inflicted in unknown.
Within a year, references to drinking metaphorical poisoned Kool-Aid began to appear in print. (Kool-Aid is better known in the United States than Flavor-Aid, which no doubt explains the substitution.) A specific reference to Jonestown appears in this quote from the September 23, 1985 edition of the Washington Post: "What he didn't want [Rep. Foley said] . . . was 'what I call the politics of Jim Jones, that "let's drink the Kool-Aid" kind of downer.'" Drink the Kool-Aid most often refers to someone's wholehearted commitment to a social or political cause. However, it can be used to warn against any kind of gullibility. For instance, a friend might warn a would-be investor, "Don't drink the stock market Kool-Aid."