The word "AMP/AMF" was first used in the Philippines by the time Ragnarok Online is on it's boom. It is a shortened version of the Filipino phrase "ang puta!/amfuta!" which means "the bitch!" used by Filipinos to express anger and dissapointed on a situation or in some cases just to add intensity to a statement. It is also used the same way "damn!", "shit!" words are used by the Americans.
puta - bitch
ang puta - the bitch
am puta - the bitch (variation)
amp - end result
putang ina - bitch mother
tang ina - bitch mother (variation)
Since, "ang puta!" or "putang ina" are considered bad words or a swear phrase in the Filipino culture
swear words: used putang ina mo! (your mother is a bitch)
it is filtered in the game Ragnarok (pRO) to prevent swearing/insulting other players.
And the "amp" word was born as a workaround to express the phrase without being filtered by Ragnarok Online.
Boy 1: tol, bagsak ka daw sa subject mong Physical Education (dude, I heard you failed in your subject Physical Education)
Boy 2: ang puta!/amp! pano ko bumagsak dun? (The Bitch! How did I failed that?)
Boy 2: putang ina naman! (Bitch mother!)
Boy 2: tang ina talaga! (Real Bitch Mother!)
Girl: ang gwapo talaga nya, amf! baka magbading ka pagnakita mo sya! (he's so handsome, bitch! you'd be a gay if you saw him