In honor of our state holiday, Pioneer Day (though it's a day late):
Pioneer traces back to the Anglo-Norman peoner, originally meaning "pedestrian" or "foot soldier." This comes from the Old French peon, which apparently has the same meaning (the -ier suffix designates a profession). This in turn evolved from the Post-Classical Latin pedon, which meant "person with flat feet" or "foot soldier." The ped- portion is simply the Latin word for "foot."
Pioneer came to mean a specific kind of foot soldier, namely the kind that went ahead of the main body of troops to dig trenches and mines, fix roads, and that kind of stuff. In other words, they prepared the way for those who followed.
In another dialect of French, peon became paon, which was borrowed into English as pawn. This word also originally referred to foot soldiers, but it came to be associated with the foot soldiers in chess, and by extension, foot soldiers who are sent to die or who are manipulated by higher powers.
And then, of course, there's just plain old peon, which apparently comes from two sources in English—there's the now-rare sense of foot soldiers or lowly officers, which comes from French, and then there's the sense of unskilled, menial workers, which comes through Spanish.
