I thought you were a lawyer and up on all the nuances of language 
A dirk is a knife made for stabbing. So they tend to be sharp and pointy - also called a thrusting dagger. They maybe be sharp on the sides, but don’t have blades that one typically associates with cutting something. They usually have small or no hand hard guards.
Like a lot of things, there is no hard and fast specification - and so you can find things that one person might call a dirk, another a dagger, another a knife. When does the length go from dagger to knife? Can a long knife be a dagger, or does it need two sharp edges?
Fun with legal language:
