A narrow dagger used in medieval times to dispatch a seriously wounded knight.
From a Latin word for mercy. Has a couple of other meanings: a relaxation of monastic rules, and a sort of leaning post in a cathedral. The leaning post one is the only definition given in a 2006 Miriam Webster dictionary; the other two can still be found in my 1982 American Heritage. No, I don’t collect dictionaries. The point is, over time we occasionally lose these nuances of language that offer a window into history.