Here’s what I learned today: “In architecture, the apse (Greek αψις (apsis), then Latin absis: “arch, vault”; sometimes written apsis; plural apses) is a semicircular recess covered with a hemispherical vault or semi-dome.” That’s from Wikipedia. Here’s what one of them looks like.
I looked this up because it occurred to me that from an engineering standpoint, building a dome like that our of large hard pieces of material–stone, probably– could be quite dangerous. Very much of that stone falls on people, some of them are going to die.
So then I got to wondering if the expression “killer apse” (often misspelled) comes from these early architectural innovations. Not very likely, I suppose.