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.