Short for "Lick My Sack," or "Lick My Swag" for a more gender neutral equivalent. More often than not, this is not referring to the actual act of fellating, but rather to virtually fellating someone. A Facebook friend might request that you lick their swag, thus bringing their e-penis to a more erect and noticeable standing. The result of complying with an LMS request is generally the expulsion of a wad of volatile links to things that the recipient thinks are interesting, as well encouraging more future requests. It is generally used by those who are desperate for attention (see attention whore), real or imagined.
"Dude, the other night I was on facebook and there were a bunch of people who told me to LMS."
"Did you do it?"
"Nah, there were so many I would have chaffed my tongue."