Rolling On The Floor Like Draco Malfoy
This term refers to the character Draco Malfoy from A Very A Potter Musical, the fan-made parody musical by StarKid Productions. In the musical, Draco is known for rolling around on the floor, so, to ROTFLDM is to roll around in an imitation of Draco.
Person 1: My friend just said she was a Hufflepuff, and I said "What the hell is a Hufflepuff?"
Person 2: ROTFLDM