You're probably thinking of the misericorde, a long, narrow-bladed dagger with similarities to both the earlier rondel dagger and the later stiletto, appearing around the 13th century and carried as a sidearm. Its shape certainly lent itself to penetrating armour to deliver a coup de grace (the name misericorde derives from a French or Middle English word meaning an act of mercy) but it wouldn't have been used exclusively for that - rather, it was a general-use knife that happened to be particularly good at that.

As ever when dealing with this sort of thing, bear in mind that mediaeval weapons were no respecters of taxonomy; the same thing could be called by different names at different times and places or by different people, or different things called by the same name.

The record is patchy on details - much of what we know about mediaeval weaponry comes from "manuals" and treatises by people like Fiore dei Liberi and Sigmund Ringeck, most of which were written a little later (15th and early 16th century) and whose audiences were understandably more interested in how to win a fight than in the messy details of what happened afterwards. There are a few mentions in Charles Boutell's Arms and Armour in Antiquity and the Middle Ages (1902!) but it's fairly superficial stuff like where they were worn and what their pommels were shaped like, i.e. details gleaned from historical illustrations, not a description of actual use.