A term used in Diablo 2 in which player(s) constantly make games, kill a particular monster(usually superunique or a boss), and make a new game in the hopes of finding "l33t itamz."
Players that do mf runs usually devote hours of time to it, often neglecting family, friends, school work, etc. These individuals neglect to exercise the ability to play cooperatively with others players, usually stealing drops from others when they do.
They don't get laid either.
He didn't answer the phone all day! He was probably jerking off while doing mf runs. What a bitch.