Abbreviation of "Long my life out".
Used to inform somebody, usually via text message, that they are taking forever to do something. Using the abbreviation implies that you are becoming impatient, thus, can't be bothered with writing the full words out.
Jack: "Are you ready to leave yet?"
Josh: "Yeah, I'm leaving in 2"
Jack: "I wasn't born yesterday, you're probably still washing your hair!"
Josh: "Haha :P"