This is a post. Like the the 706 others before it.
If you look at one of the vertical parts of a fence, that’s a post.
If you have a job, that’s a post.
If you send someone a letter, that’s post.
Why is the word post so multi-talented?
There are two basic meanings involved.
The first is from the Old English post, meaning pillar or doorpost. The second is from the Vulgar Latin positum, meaning put (in the past participle form).
The second is where we get post in the sense of job/position, mainly from its military use.
The first has led a few different meanings of post. There’s obviously post as in fencepost. It’s also where we get the word post as in blog or Facebook etc. post. This comes from the use of to post as a verb, meaning to affix something to a post. Eventually this evolved into post also referring to the thing posted to the post. Post. And this meaning became pretty naturally applied to online publishing, as you post something to your blog or social-media account like the way you post something to a noticeboard.
The second meaning is where we get post in the post-office sense from. This comes from 16th-century France, when riders would be posted at intervals along a route to speed up the delivery of mail in a relay system.