Stands for "I really don't know." A version of the word idk, or "I don't know." Used in chat rooms. When you're really at a loss for an answer to a question is the best time to use it.
Shannon: So, what did you think of that Chuck Norris horror movie?
Billy: Irdk. But it scared the shit outta me.
Billy: Irdk. But it scared the shit outta me.
by SaraBellz July 6, 2008