The words pen and pencil appear to be related, but are they? Let’s find out.
The word pen, as in a writing implement, comes from Middle English penne (pen, quill, wing, feather), from the Anglo-Norman penne, from Latin penna (wing, feather, quill pen), from Proto-Italic petnā (feather, wing), from Proto-Indo-European *péth₂r̥/pth₂én- (feather, wing), from *peth₂- (to spread out, to fly) [source].
Words from the same roots include petal, petulant, petition, plume, plumage, fathom, feather and helicopter in English, adar (birds) and adain (wing) in Welsh, and Faden (yarn, thread, fathom, suture) in German [source].
The word pen, as in an enclosure for animals, comes from Middle English pen(ne) (enclosure for animals), from Old English penn (enclosure, pen, fold), from Proto-Germanic *pennō/*pannijō (pin, bolt, nail, tack), from Proto-Indo-European *bend- (pointed peg, nail, edge).
The English word pin comes from the same PIE root, as does the Dutch word pin (peg, pin), the German word Pinne (pin, pivot, tiller), and the Swedish word pinne (stick, peg, pin) [source].
The word pencil comes from Anglo-Norman, from the Old French pincel (paintbrush), from the Vulgar Latin *penicellum, from the Latin pēnicillum (a painter’s brush, (style of) painting), a diminutive of pēniculus (brush, sponge), a diminutive of pēnis (tail, penis), from the Proto-Italic *pesnis, from the Proto-Indo-European *pes-ni-s, from *pes- (penis) [source].
Words from the same roots include penicillin in English and other languages, pincel (paintbrush) in Spanish and Portuguese, pinceau (paintbrush) in French and Pinsel (paintbrush) in German [source]. Penicillin and penicillium are apparently so named because the spore of the fungi resemble brushes [source].
Incidentally, the French idiom s’emmêler les pinceaux means to get one’s wires crossed to get things all mixed up, to get in a muddle or to misstep. Literally it means “to get tangled in the paintbrushes” [source].