An abbreviation of Mammaries Or Fare Thee Well. This is a variation of the more uncouth and possibly not as clever expression tits or GTFO, which is designed to quickly explain to newcomers on internet forums or imageboards that they are only tolerable if they post pictures of themselves topless, otherwise the established members would like them to leave.
Similar to COTW, this stands for "Monster Of The Week" as used by fans of TV show Supernatural. In earlier seasons all episodes were MOTW, but now with Apocalypse storylane, it's used to indicate filler episodes.