(n.) the topmost brick at the gable end of a house
While the stone at the peak of an archway is the keystone, the brick at the peak of a house’s gable end is the crowstone.
Or, at least it is according to a number of regional and dialect dictionaries, including James Orchard Halliwell’s monumental 1855 Dictionary of Archaic and Provincial Words. But why crowstone?
That’s a good question—and one that none of those regional and dialect dictionaries (nor the Oxford English Dictionary, for that matter) sheds any light on.
We can at least put forward a couple of suggestions here, however. For one, it’s possible that ‘crow–’ here is a corruption of another word—perhaps crop, or crope, a term that has long been used of a decorative flower-like finial atop the peak of a roof pillar.(Etymologically, this crop is the same one that is harvested at the end of the growing season: derived from the Old English cropp, it referred to the headmost or topmost part of practically anything at all, long before it came to be almost exclusively attached to only the topmost part of a cultivated plant.) But then again, perhaps this ‘crow–’ is just, y’know, a crow.
Because gable ends typically form high and noticeable peaks, their topmost parts are popular perching sites for birds wishing to assert their vocal dominance over their territory. This alone might have been enough to earn them the name crowstones—but perhaps the crow’s association with the darker end of folklore in particular might have made their name particularly meaningful here. A crow or similar bird seen cawing from the roof of a house has long been considered a sign of ill portent, so perhaps that dreaded sight was enough to establish an etymological connection.
But with little textual evidence outside of dictionaries to go on, alas, these will have to remain nothing more than theories.