From my own research, I personally believe that “dirk” is an obsolete 19th-century American term that probably never did have a clear meaning. Cutlery catalogs from the late 1800s seem to have used it as a catchall phrase for any type of knife that was being marketed for self-defense. A close examination will uncover Bowies, hunting knives, small hide-out daggers and single-blade folders of all sizes being listed as “dirks.”
“Gravity knife” is another widely used term that has no universal definition. Knife collectors would probably think it should only refer to the out-the-front type openers carried by German paratroopers. On the other hand, New Yorkers have learned that in their city it can mean just about any folding knife that local law enforcement agencies choose.
Closer to home, several of the Puget Sound urban areas have placed their own local blade length restrictions on pocket knives. Try moving around western Washington without going up or down the I-5 corridor through one of these areas. Our very liberal knife laws in 90% of the state are not going to help you if you are stopped in Seattle.