also pen-knife, "small pocket-knife," early 15c., penne-knif, from pen (n.1) + knife (n.). So called because such small knives were used to make and mend quill pens.
Entries linking to penknife
pen n.1
late 13c., penne, "writing implement made from the hard, hollow stem at the base of a feather," from Old French pene "quill pen; feather" (12c.) and directly from Latin penna "a feather, plume," in plural "a wing," in Late Latin, "a pen for writing," from Old Latin petna, pesna, from PIE *pet-na-, suffixed form of root *pet- "to rush; to fly."
In later French, this word means only "long feather of a bird," while the equivalent of English plume is used for "writing implement;" the senses of the two words in French thus are reversed from the situation in English.
In Middle English also "a feather," especially a large one from the wing or tail. The sense was extended to any instrument of similar form used for writing by means of fluid ink. Pen-and-ink (adj.) "made or done with a pen and ink" is attested from 1670s. Pen name "fictitious name assumed by an author" is by 1857 (French nom de plume was used in English from 1823). Southey uses pen-gossip (v.) "to gossip by correspondence" (1818).
knife n.
"hand-held cutting instrument consisting of a short blade and handle," late Old English cnif, probably from Old Norse knifr "knife, dirk," from Proto-Germanic *knibaz (source also of Middle Low German knif, Middle Dutch cnijf, German kneif), a word of uncertain origin. To further confuse the etymology, there also are forms in -p-, such as Dutch knijp, German kneip. French canif "penknife" (mid-15c.) is probably of Germanic origin, perhaps from Frankish. For pronunciation, see kn-.