You Call That a Knife? THIS Is a Knife!
So for now, we'll just say, it's a weapon and it's cool.
The Messer rose to prominence in the late medieval era, created as a slashing-weapon similar, if not identical in function to the Falchion. In Germany, this was by and large the most common weapon used, as they were relatively cheap and happened to skirt a law that allowed only knives to be carried by commoners. Remember when I said this weapon was kind of a weird...knife/sword hybrid? Well, that's because the Messer is technically classified as a longknife rather than a sword.
...to see that it's a far cry from the simplicity of the Messer. Note the lack of a fuller or rain-guard, the emphasis on cutting rather than stabbing, and the single edge. All characteristics commonly associated more with European knives than with swords.
So essentially, what you had was a weapon that functioned like a sword, but legally speaking, was every bit as much of a knife as the apple-peeler in your back pocket. So to skirt the draconian laws that kept peasants in their place, lower-class Germans would arm themselves with knives that were technically swords, but fit all the legal minutea of what a knife ought to be.
Except...it's actually not true. Everything I said about law-skirting German peasants is, in actuality, all a lie. A commonly accepted lie, one even noted HEMA youtuber Lindybeige stated in his video on the subject. And in basically every source I looked up on the subject, the Messer's status as a "longknife" was chalked up to the "Germans weren't allowed to have swords" explanation. This is in fact, not true however. As it turns out, Germans in that period were required by law to own actual swords. As in, "Hippies without swords will get thrown in the goddamn dungeon because real men own swords" illegal. "We expect a standing militia, so stop listening to Bob Dylan and AMASS AN ARMORY TO DEFEND YOUR FAMILY AND COUNTRY" illegal.
Crazy, right? This is basically the exact opposite of everything I read! And yet, there's actual academic proof for the latter. Proof covered by another fantastic HEMA youtuber, Shadiversity who explains why exactly the Messer has such a complicated status in the weapons world. Long story short, the most historically plausible belief happens to involve knifemakers wanting to produce swords, and skirting the laws of their own guilds to produce weapons that were technically knives, but functioned like swords, so as to get a leg up over those jerkwads in the Swordmakers' Guild.
Below is Shad's lengthy and quite informative five-part video series on Messers and Falchions from an academic standpoint. If like me, your lust for knowledge of all things sword-related cannot be contained, I suggest you give it a watch.
Also, consider subscribing to him! He's a fantastic content creator, and utterly essential if you wish to implement historical realism in your fantasy narrative, be it a novel, D&D session or whatever else.