If you can say bodkin, perhaps this is something you already knew existed, but couldn't recall the name? In any case, you're thinking of a stiletto. There are plenty of websites offering repros, some of which are pretty fanciful, but this one is likely to be the real thing. Not that it would pierce iron plate, but could fit in between the joints, or even get past chain mail.

They normally had triangular, sometimes square cross-section blades. The French name was miséricorde, the implication being that once an opponent was lying wounded, the knife would quickly finish him off and put him out of pain.