“Drinking the Kool-Aid” is a metaphor commonly used in the United States and Canada that refers to a person or group holding an unquestioned belief, argument, or philosophy without critical examination. The phrase typically carries a negative connotation when applied to an individual or group. The basis of the term is a reference to the November 1978 Rev. Jim Jones Jonestown Massacre, where members of the Peoples Temple were said to have committed suicide by drinking a Kool-Aid drink laced with cyanide.
Objections notwithstanding, the phrase has been used in a variety of contexts to describe blind, uncritical acceptance or following. This usage began in limited circles in the late 1990s, and reached mainstream use in the late 2000s.
I’ve heard people use this expression and had no idea where it came from. Now you know