NOTE: If said person is dumb enough to use IDN, I doubt they'd take the time to use capitals, making it "idn."
You may also see "IN", or "in," which can translate into the opposite: "I know," or to an educated abbreviator, "ik. "
Second dumb person: idn i here hes pretty dum
First dumb person: but hes so hott
Second dumb person: ya in lol
First smart person: Hey, Britney, do you like Jake?
Second smart person: I don't know, I hear he's pretty dumb.
First smart person: But he's so attractive.
Second smart person: Yeah, I know.