So what picture did you get in your mind? Maybe an object with short blades that fold up out of a protective cover? Or a Swiss Army knife with seventeen other bits and bobs on it beside a blade or two?
Not much like today’s penknives, is it? That’s because it wasn’t. In the 19th century, a penknife was exactly that—a knife used to make or trim pens.
Up until the mass production of the steel-nib dip pen in the early 1820s (though metal pens had been around since the early 18th century), the chief type of pens in use were made from quills, usually the primary flight feathers of large birds like geese, hawks, eagles, or turkeys. They actually make good pens; the hollow shaft of a quill serves as an ink reservoir, and a properly prepared quill can last a long time (though not as long on today's wood-pulp based paper which can quickly wear down tips; they do best with parchment and vellum.)
There’s a great explanation of how to trim a quill here, for the truly curious...and it explains the fine, thin shape of a penknife’s blade since the blade was inserted into the quill at one point. Note on the photo of my penknife the shape of the handle—curved to fit comfortably against the fingers when shaving off bits of quill.
So that’s what a penknife is. And though I’m very happy with my laptop for writing, I can’t help sighing a little for the pretty desk accoutrements of the nineteenth century. Maybe I could find someone to make me a pretty mother-of-pearl cover for my mouse?