1) A folding knife to fit in the pocket. Originally the blades opened at one end only.
The few OED references are late and the term has not been noted in local records until the 1700s. In 1721, the inventory of John Woolin of Sheffield listed 1 doz. horne hafted pocket knives worth 1s 6d and 3 doz. bone penknives with the same value. Today the terms ‘pocket knife’ and ‘penknife’ are used interchangeably.