Refers to IQ; any member of the double digit club has (or is assumed to have) an IQ of 99 or below, ie below average. Almost exclusively used as an insult.
"Hey, Becci, you're really a member of the double digit club."
"Huh?"
(Several smart people around Becci start laughing).
"Huh?"
(Several smart people around Becci start laughing).
by Overand February 12, 2005