Being a crack is simply being brainless and idiotic.

One who has a mania for music.

before it was associated with any type of drug. Every one knew that "a crack," was corporal punishment doled out usually from a teacher or school principle at the end of a paddle or other type of manufactured board and applied to the misbehaving students posterior ( butt crack) this was well known slang (1960's-early 80's)