"Drink the Kool-Aid" is a slang term originating in United States, and connoting unthinking acceptance of propaganda or a cult of personality.
The phrase originated in the mass homicide/suicide that took place at the Reverend Jim Jones' People's Temple, when devotees of his cult drank Kool-Aid laced with cyanide.
The first reference to the phrase "Drink the Kool-Aid" was entered on urban dictionary in 2005, and it seemed to become a more widely used catch phrase around the time of the 2008 Elections. Although the origins of the phrase were highly sinister, indicating slavish devotion to a charismatic leader, it can be applied to much milder devotions, for example, someone could be said to be "Drinking the Kool-Aid" when being a fan of using Apple products.
The phrase came into popular use thirty years after the events that inspired it. Most people who use the phrase probably either were not yet born when the Jonestown Massacre happened, or at least wouldn't have had a clear memory of it. Perhaps this distance is needed: while parts of the Jonestown Massacre seem almost silly (especially the memetic Kool-Aid), it was also an event where almost a thousand people died. It is probably the distance of time that allows people to use it as a slang term without fully acknowledging its horrific origin.