Source : Webster's Revised Unabridged Dictionary (1913)

Penknife \Pen"knife`\, n.; pl. {Penknives}. [Pen + knife.]
   A small pocketknife; formerly, a knife used for making and
   mending quill pens.

Source : WordNet®

penknife
     n : a small pocketknife; originally used to cut quill pens
     [also: {penknives} (pl)]