This popular reference refers to the specific suicide method (Purple Kool-Aid)used by the 900+ participants of the People's Temple cult in the 1970s ran by Jim Jones.

A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.

As heard in the Incubus song "Purple Kool-Aid"

Refers to poisoned Kool-Aid used in cult ritual suicides.

Purple Kool Aid is the liquid that keeps Black People alive. It is the equivalent of water to White People.

Drinking the Purple Kool-Aid will cause a person to blindly follow those that gave it to them. This definition is considered to be a misinterpretation of the Purple Kool-Aid incident in 1970.

Something that will make you hyperactive, and what African Americans Drink.

as heard in the Incubus song "Purple Kool-Aid"

there are many speculations as to what purple kool-aid could be. for example it could be a concoction of LSD, tranquilizers and purple kool-aid. or it could be a street name for a hallucinagenic drug. or it could be as simple and innocent as standard purple kool-aid.