LLACP

LLACP stands for "laughing like a crazy person." If a friend sends you a joke or meme that sets you laughing uncontrollably, you can send back LLACP to let your friend know their joke shattered your mind.

If you want to convey the same amount of amusement in fewer letters, you could instead tell your friend that you're ROFL, CSL, or LWT. However, there's a certain level of "I could get locked up in Arkham Asylum because I'm laughing so hard"-ness that's captured only by LLACP.