(i) Mother Fucker
(ii) My Friend
definition (i) is more commonly seen in chatrooms; whereas definition (ii) seen in multiplayer games.
if someone uses mf towards you, it's best to pretend you dont understand, so they will explain - either (i) or (ii). Since you dont want to just start insulting them.
<Shinosuke> Blah.. lets stop the conversation.. I dont want to talk abt it anymore.
<Person1> ok mf.
<Person1> mf - my friend.
<Shinosuke> Oh.. ok.. i thought.. nvm.