This is an excellent question! The expression comes from the infamous mass suicide at the People's Temple under the leadership of Jim Jones in 1978. Koolaid is a brand of powdered soft-drink mix. On this fateful day, Reverend Jim Jones convinced his followers to drink Koolaid laced with poison in order to prevent members from leaving the cult. https://en.wikipedia.org/wiki/Drinking_the_Kool-Aid The expression is now used to describe someone who believes what is being told to the public by the mass media or government officials without investigating whether it is is true or not.

"Drinking the Kool-Aid" is an expression commonly used in the United States that refers to any person or group who goes along with a doomed or dangerous idea because of peer pressure. The phrase often carries a negative connotation when applied to an individual or group.

It is a reference to a horrific incident in history. It refers to something that really happened. The incident was "the Jonestown mass suicide" of 1978 in which over 900 people drank poisoned Kool-Aid and died. If I tell someone that I prefer a Macintosh computer to Windows, they might say "You've drunk the Apple Kool-Aid. Windows computers are cheaper and just as good." The idea is that I believe anything that the Apple Corporation says and do anything they tell me to do--just like the people in Jonestown. Kool-Aid is the brand name of a US soft drink product. It is a dry flavoring powder that comes in envelopes. You mix it with sugar--lots of sugar--and water. It comes in different flavors. It creates a brightly colored sweet drink with a strong, very artificial fruity taste. It is very cheap to use. It is "liquid candy" and not healthy. It is very cheap and often used when serving large groups of people. The Wikipedia article on "Jonestown," https://en.wikipedia.org/wiki/Jonestown , tells about the incident. A leader founded a community of followers. They created a settlement in a remote area in Guyana. When things fell apart, the leader convinced over 900 of his followers to commit suicide by drinking Kool-Aid poisoned with cyanide. Cyanide has an almond-like smell and the strong flavor of the Kool-Aid probably helped cover it up and make it easier to drink.

It means to blindly accept false doctrine or go along with a bad idea because of peer pressure, often with negative consequences. The origin is when a cult leader made the followers drink "kool-aid" (which is typically a sugary, fruity drink that children enjoy) which was poisoned. I've heard this expression most often when people believe promises made by the government or some other big organization.