Stands for "Eat My Shit" (NOT Emergency Medical Service). See also, "S.M.D.," which stands for "Suck My Dick". Both acronyms are useful in social siutations where you wish to make an urgent, direct point; but full expression might be inappropriate, embarrassing, or just plain bad for your health or well-being.
Hey, you over there by the casserole, E.M.S.!