Abbreviation for "In Real Life I Don't Know You", most commonly used in text messaging and sexting as an excuse not to give out personal information.
"Hey Billy! Send me dem n00ds!"
"No way! IRLIDKU!"
"You could know me if you wanted.."
"Billy, I'd love to come over tonight but my mom won't let me because IRLIDKU. She says you could be an internet predator."