"Hartshorn" is a (perhaps old-fashioned) term for a particular kind of smelling salts (using ammonia); MW11 says it's because harts' horns were once the main source for ammonia.
Another one from Privilege of the Sword.
The term seems rather evocative to me; it sounds like it should be the name of a lonely castle on a moor somewhere. (Especially because I keep wanting to mispronounce it as "heart-shorn.")
(I'm obliquely reminded that it took me a while, as a kid, to parse the title of the Peter Dickinson novel Heartsease; I think I kept wanting it to have something to do with seas instead of ease.)