the.com/collar stay

means A small, stiff strip of plastic, metal, or bone that slides into a pocket on the underside of a shirt collar to keep its points flat and straight.

from Straightforward trade vocabulary: 'stay' has long meant a supporting rod or brace (as in corset stays, which rigidified Victorian-era bodices), from Old French 'estaye,' meaning a prop or support, itself from a Germanic root tied to standing firm. Collars got the same treatment once detachable, starchable shirt collars became fashionable and needed help resisting curl — the stay is just that old bracing logic, shrunk down and tucked into cotton.