I thought it had something to do with church architecture, but no. Not the most common word ever, but it does appear in multiple collegiate dictionaries, so not super-rare either. Borrowed in 1584 from Middle French periapte, from Greek períapton, amulet, from períaptein, to fasten around (oneself), from perí-, around + haptein, to fasten.
Almost instantly I discovered that my uncle's periapt was missing.
(Yes, that's a real quote, from one "Adventure of the Purloined Periapt". I have not attempted reading it.)