Yes, another multiparter, and this one will be at least three. There are a lot of words with pose/position in them. This will actually take me through all of August up to my birthday, which, I don’t know, I guess is something.
Pose first showed up as a verb in the late fourteenth century as posen, where it only meant suggest, assume, or concede. It wasn’t until the nineteenth century that it meant pose like you would a body, which was influenced by the French usage of the word. Pose (or posen, rather) comes from the Old French poser, place or propose, and LateLatin pausere, rest or cease (and also gave us pause, by the way). And weirdly enough, the word position isn’t related to it. It showed up in the late fourteenth century as a term in logic and philosophy, coming from the Old French posicion, position, and classical Latin positionem, also position. That word is actually from the word ponere, to place but apparently the French conflated the Late Latin’s pausere with the classical Latin ponere/positionem and that stuck all the way through English. And now words that should end in -pone end in -pose.
Oppose first showed up in the late fourteenth century, coming from the Old French oposer, oppose, as well as poser, the same word from above. Which means of course, that there technically is no etymology beyond that. Poser was combined with the classical Latin opponere, to oppose, which is the prefix ob-, against, and poner, to place. So that makes this word “to place against”, which sounds like oppose to me. Also, opponere is also the origin word for opponent, where the etymology actually makes sense.
Next, expose showed up in the early fifteenth century, coming from the Old French exposer/esposer, which could mean lay open, set forth, or speak one’s mind. Like the other -pose words, its Latin word has a different etymological line. It’s exponere, to explain, a mix of the prefix ex-, meaning forth in this case, and ponere, to place. So it’s to place forth, just with words, which kind of makes sense. And like with opponere, exponere’s etymology is more clearly seen in other words, like expound and exponent.
Suppose showed up in the early fourteenth century meaning “to assume as the basis for an argument”. It comes from the Old French suposer, to assume, which actually had an earlier (and correct) form in suppondre. Further back it’s the classical Latin supponere, which had a variety of meanings, from suppose to subordinate to place under. The sup- comes from the prefix sub-, under, and ponere, place. So the Latin uses of the word kind of make sense—subordinate is literally stand under—although I’m not sure how the other definitions came about.
Finally today, we’re looking at purpose and unlike the other words, it is not a Latin word confused by French. In fact, it’s pretty much entirely of French origin. It showed up in the early fourteenth century, from the Anglo French purpos and Old French porpos, intention. Porpos comes from proposer, to put forth, a mix of the French prefix por- (which comes from the classical Latin pro-) and poser, to place. So “to place forth” becomes to put forth, which somehow turns into intention. Nope, no idea. But it’s interesting that purpose is the only one of these words where the accent is on the first syllable. I wonder if it’s the French origin.
TL;DR: All words that end in -pose should really be -pone because Latin words, but aren’t because of French.