The background of
the wicked shit didn't start with ICP but with
Esham The wicked clowns, ICP, used the wicked shit to spread the word of the Dark Carnival. Esham The Boogie Man taught the two the "secrets" of the wicked shit and helped them get the underground cred they needed.
The wicked shit is what is flowed from any underground hardcore rapper or rocker. Psychopathic Records seem to have collected the best at using the wicked shit. Esham, Blaze, ICP, ABK, and Twiztid. The wicked shit was used to "hide" the message of the Dark Carnival being GOD, since not many of the juggalos or juggalettes would have followed them he they dropped the hatchets and gats.