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).
military slang for someone who is "short" on a tour of duty, less than 100 days away from going home, or retiring.
Tim retires in June, so in February he will be an official double digit midget. Countdown to never wearing a uniform again!! 100, 99, 98, better than Christmas!