I can’t believe it took me this long to get around to pronouns. Common words like that are bound to have weird origins.
First let’s look at I. Now, I went over this when I did my look at the letter I, but that was eight years ago, so it’s time for a refresher. I the pronoun showed up in the twelfth century, a shortening of the Old English ic, which was actually pronounced itsh. Not kidding. It’s from the Proto Germanic ek, from the Proto Indo European eg-, which again is just the pronoun I. But not the letter I.
Me comes from the Old English me, but once upon a time there was also mec, which was the reflexive and accusative form, while me was accusative or dative. Yeah, they were both accusative. I’m just glad they only have one now, because I would hate having to figure out when to use me and when to use mec. My showed up in the thirteenth century as mi and was a short form of mine used before words beginning with consonants—except H. It wasn’t until the fourteenth century that it was used before all pronouns. Mine—the pronoun, not explosive mines or dug mines, obviously—comes from the Old English min, which was pronounced more like the first half of minute. My just became more popular to use, for some reason.
Next, we comes from the Old English we, from the Proto Germanic wejes and Proto Indo European we-. How shockingly straightforward. Us is similarly not totally insane. It comes from the Old English us, which actually would have been pronounced more like oos, and is traced to the Proto Indo European nes-. Yes, it once had an N! That’s why lots of other European languages have some variation of “nos” for their forms of we/us/our. There’s no reason why English dropped the N. We just did. And speaking of our, it comes from the Old English ure, from the Proto Germanic ons, which is also from the PIE nes-. We also lost the N on that one, although Middle English did have ourn and ouren as the plural of our. I think we were just lazy about pronouncing Ns.
Finally today, it. It comes from the Old English hit, yes, with an H that people just stopped saying, and it should be noted that back in Old English, it was the gender neutral pronoun to go with his and her. But then in Middle English, it started referring to an object rather than just being neutral, which gave rise to everyone using they to be neutral and man, is that a whole thing. Anyway! It is from the Proto Germanic khi-, from the Proto Indo European ko-, which means this and is the origin for a bunch of words, like hither and hence. But remember, just because it was once gender neutral doesn’t mean you can get away with calling a person that now!
Tony Jebson’s page on the Origins of Old English