Someone who believes what they are told without thinking for themselves.
Sometimes used by aficionados of the 1911 handgun ("Colt
.45") to describe fans of the Glock
(or other polymer framed) handguns - possibly a reference to the nickname "tupperware gun" and the typical serving of Kool-Aid in plastic pitchers.
In more recent use it may also be applied to folks who think that any negative term or euphemism applied to President Obama or his acolytes automatically means the user is a racist.
The suggestion that the term applies only to a stereotype of African-Americans implies that no Caucasians drink Kook-Aid, an obvious fallacy and an indication of a significant pre-existing bias on the part of the writer, or a blatant attempt to suppress freedom of speech on the part of opponents.
From the followers of Jim Jones in Guyana who committed suicide by drinking poisoned Kool Aid.
The Kool Aid drinkers are buying this stock because their broker recommended it.
A Glock? What are you, a Kool Aid drinker?