Pacifism is historically almost the primary and first type of recognizable Leftism - with the Quakers - but I think that psychologically Pacifism derives from already-established egalitarianism.
I take it that pacifism, as a political principle, is about men rather than women (since women are usually exempt from war) - and my intuition is that it derives from genius men.
In historical societies, I think the equivalent people to modern geniuses were shamans, priests, healers, advisers etc. In sum, it was recognized that geniuses were unsuitable for deployment as soldiers.
Psychologically, part of being a genius is a primary commitment to the importance of one's own work, above social ties and duties. So the genius feels, about himself: "I personally will not fight."
But when egalitarianism began to get a grip, geniuses saw that the logic was that either all should be conscripted for war, or nobody - and this, I think, led to pacifism.
In effect, the genius's own imperative to avoid war - plus egalitarianism - drove him to advocate pacifism as a universal principle; on the basis "I personally will not fight, but all should be equal - so what applies to me should apply to everybody; therefore nobody should have to fight".
I believe that this is the basis of genuine, principled (albeit wrong) pacifism.
Such geniuses constructed pacifism as a political principle - rationalized in various ways; and various other - differently motivated people - subscribed to pacifism until they made a vast majority.
But the origin of pacifism is, I believe, in the combination of genius with modern (i.e. post-medieval) egalitarianism.