UMMLSFM is yet another annoying internet slang term that forces us to take minutes out of our lives to decrypt; but alas, it has been done. UMMLSFM, or "you make me laugh so fucking much", is often used in pair with worship of someone or someone you wish you were, but you are not. It also tends to make the person who uses it look like a faggot who's trying to start some new trend.
wannabe internet hipster: "lyke, zomg lmfao, ummlsfm"
someone intelligent: "I don't know what the fuck you just said."
wannabe internet hipster: "rofl!!!"
someone intelligent: "You're an asshole."