See, I said I was going to do to sit, and now I am. And there are a lot of words related to this one. Yay, another multi-parter!
Sit itself comes from the Old English sittan, which just means seat, so no shocking revelations there. It’s from the Proto Germanic setjan, from the Proto Indo European sed-, to sit. Isn’t it weird when it’s straightforward?
Seat unsurprisingly is from the same place, although it has a kind of different way to English. See, it showed up in the thirteenth century from the Old Norse saeti, seat. So, slightly different origin. But that too is from Proto Germanic, in this case the word saet-, which is also from sed-. Also, seat kind of evolved by the late thirteenth century to also mean an established place (i.e. a seat of power), and because that could also be a residence, there are lots of English place names that end in -set. Probably the most interesting use of seat is in deep-seated. That word is much more recent than the other seats, having shown up in 1741 meaning “having its root far below the surface”. Since seat could mean an established place, a deep seat was a firmly established place. And that’s the reason for that.
Set is also related, and also brings in the weirdness of etymology we’ve all come to love. See, to set, as in to set something down, is from the Old English settan, to set, establish, or place. It’s from the Proto Germanic bisatejanan, to sit/set, and that’s from the Proto Indo European sod-, a variant of sed-. But. A set, as in a set of items, is not related. It has a completely different origin! It’s from the Old French sette, sequence, a variant of secte, religious community. You know, like sect. It’s from the Medieval Latin secta, retinue, from the classical Latin secta, a following. Yeah, a set of items is a sect, not a set (down).
Finally today, we’ll look at settle. It comes from the Old English setlan, to cause to sit, from setl, a seat (its definition actually ranged from a chair or throne to a butt!). It’s from the Proto Germanic setla-, from the Proto Indo European sedla-, another offshoot of sed-.
A settle is a butt. I find that way too amusing.
Tony Jebson’s page on the Origins of Old English