Amf is actually "PUTANG INA MO"'s shorthand version. It was first seen on the popular game "Philippine ragnarok Online", now it is used in IRC and IM (Yahoo Messenger)
Amf has two uses:
1) To Offend / Insult someone
2) As an expression
See also: Putangina mo, Gago , Lol, hehe
Usage 1: pRO ingame scenario
*Player A Killed A Monster*
*Player B took the LOOT without permission *
Player A to Player B: Amf ka! Go to hell! GRRRRRRR.
Usage 2: IM Scenario
Someone: Have you heard that the Peso to Dollar rate is 60 pesos to a Dollar?
Good bye, sarcastic farewell, go away, get away & don't come back, I'm leaving & I'm not coming back, Adios, Motherfucker, If you don't believe that I'm leaving-count the days I'm gone.