A dumb and unsophisticated way of saying "I Don't Know." Usually used by lazy teenagers who spend their time going on myspace and drinking orange soda.
guy 1: Do you know what 2+2 is?
dumb teen: idk
guy 1: My house is on fire! What should I do?!
dumb teen:uhhhh...... idk....get insurance?
teacher: Karie, you wrote "IDK" on all of your test answers. Could you explain why you did this?
dumb teen: uhhh......IDK.......