mf has 2 meanings, often used as:
(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.
Example:
<Shinosuke>
Blah.. lets
stop the conversation.. I dont want to talk abt it anymore.
<Person1> ok mf.
<Shinosuke> mf?
<Person1> mf - my friend.
<Shinosuke> Oh.. ok.. i thought..
nvm.