Post is one of those words that shows up in a lot of different ways. This right here is a post. If you see a fence, I’m guessing it has posts, too. My brother is in the navy, and he’s posted in Japan. When you send a letter (is your computer broken?) you use the post office. Putting up a sign is called posting it. Also, should you need to describe something that is after something else, it would be post that something else. Okay, that description is a little convoluted, but you get the idea. And what is up with “posthaste”? Is there a reason for these eclectic meanings? I don’t know. Let’s check.
The fence post doesn’t have a date. It comes from the Old English post (…sometimes, I swear, they’re not even trying) and before that, the Old French post (okay, now I know they’re not), which basically has the same meaning. Like most French, it’s derived from classical Latin, in this case postis, which means door, post, or door-post. I’m not even kidding, that’s what Google Translate says. The origin ends there, although it’s possible that postis is a combination of the Latin words por (the prefix pro) and stare (stand). Now, this isn’t definite, but it would be interesting if it were true.
What about a post, like when you’re on duty? Is that related? Nope! That post showed up in the late sixteenth century, coming from the Middle French poste, with the same meaning. The French actually took the word from Italian—posto, meaning place—but before that, things are back to normal because Italian took it from the Vulgar Latin postum and classical Latin positum, meaning set, as you’d set something down. So it went from placing a thing to placing a soldier. That kind of makes sense.
The post we see stuck in front of modern or script comes from the Latin word for after. Which just so happens to be post, because of course it is. It can be traced all the way back to Proto Indo European apo, which means off or away.
The rest of the posts come from one or the other of these definitions. Posting a sign (or, you know, a blog post) is from the idea of nailing something to a post. The duty post is where we got the mail post from—riders posted at intervals along the mail route. Plus, the post in posthaste comes from post horses moving fast, basically telling the posted mailriders to move with haste.
TL;DR: All the posts come from three completely different Latin words that we English speakers decided should all be the same. Anyone else think that eventually, all language is just going to be one word said over and over? Because sometimes that’s what it seems like.
Tony Jebson’s page on the Origins of Old English