WIM stands for "woe is me." The abbreviated form of WIM is used online only. The phrase is used to convey to someone that you are sad, upset, and/or are in a rough spot. "Woe is me" is a somewhat archaic phrase, but it is quite useful. Use it if FML (fuck my life) is just too strong for your situation.
Girl 1: "I dropped my phone into the toilet."
Girl 2: "Really? That sucks!"
Girl 1: "Yeah, and I had just taken a crap."
Girl 2: "OMG!"
Girl 1: "Yeah. WIM. ;*("