this was a projection of my own thought process when I saw the prompt word lol.
I went on and found out that buttress comes from the old french boter, so the butt in buttress is from the verb to butt, ie. to thrust, while the human butt most likely comes from the dutch bot which means stumpy/blunt. However, both words come from the proto germanic-root butaz/butan and the proto-indo-european bhau-, so in the end language is made up and reading too much into it makes me want to butt my head against the wall :)