The more casual, edgier brother of lol. Use it to laugh at the miseries of others or to express a casual smirk that lasts between one to three seconds.
Your Friend: So I fell down the stairs today and broke my arm You: lul
the origin is of the muskrat mix, so for example when you have an australian muskrat (lol), and an Austrian muskrat (lul), you mix them together to get a hybrid muskrat, which is funnier than both original version of the muskrat. Pretty simple definition in my opinion to be honest.