This terms stands for Fun Without Lily. It indicates when a group of friends is "Lilyless," and thus, having an inherently more fun time than if there were a Lily present.
The acronym form of "Fuck Windows Live Messenger". If you'd used Windows Live Messenger for many many years, so when you gave it up because of its' errors, ads etc. this is the best expression to explain your current feelings.
-Hey dude where are you? Hurry, come to messenger, your girl'sonline!
-FWLM! I'm on Skype.