A sarcasticderivative of lmao, lma means that you know what your friend just said was a horrible joke that he is laughing, and you think it was pathetic and don't really want to laugh at it all.
Person 1: Why did the republican cross the road?
Person 2: ?
Person 1: TO KILL BABIES!! LOLOLOLOL Person 2: lma...
Person 1: *explodes*