a slang used to refer to the perineum, the part of the body between the anus and genital organs, overlying the pubococcygeal muscle. This term supposedly originates from the fact that, "'Tain't your balls, and 'tain't your ass," "'tain't" being a contraction of "it" and "ain't", in this case, meaning "it is not."
SON: "Hey Dad, how come they call it a taint?"
FATHER: "Well son, 'cause 'tain't your balls, and 'tain't your ass."
a phenomenon that occurs whenever one witnesses Shaquille O'Neal beat the shit out of several people using his Shaq Fu while advertising several products, grab one of their asses, dunk the last one into a garbage can, then roar like a t-rex
The link above includes a shaqgasm.