Origins
The English word 'pen' is, at its etymological heart, the word for a feather. It entered Middle English as 'penne' in the thirteenth century, borrowed from Old French 'pene' (feather, quill pen), from Latin 'penna,' meaning 'feather' or 'wing.' Latin 'penna' is a contraction of the earlier form 'petna,' from Proto-Italic *petna, from the PIE root *pet- meaning 'to rush' or 'to fly.' The same PIE root produced Greek 'pteron' (wing — as in 'pterodactyl,' literally 'wing-finger'), Sanskrit 'pattra' (wing, feather), and, through a different derivation, English 'feather' itself (from Proto-Germanic *feþrō, from PIE *pet-reh₂).
The semantic path from 'feather' to 'writing instrument' is straightforward and reflects actual technology. Quill pens — made from the large primary flight feathers of geese, swans, crows, and occasionally turkeys — were the dominant writing instrument in the Western world from roughly the sixth century CE to the mid-nineteenth century. The feather was cut with a small knife (a 'penknife,' a word that preserves this history) to create a nib, and the hollow shaft served as an ink reservoir. The best quills came from the five outer wing feathers of a goose, and because most writers were right-handed, feathers from the left wing were preferred, as their natural curve arched away from the writer's face.
In Latin, 'penna' meant simply 'feather' without any necessary connection to writing — the writing sense was a secondary development that emerged as quill technology spread. Italian preserves both meanings: 'penna' means both 'feather' and 'pen,' and the pasta shape 'penne' (plural of 'penna') is so named because the tubes are cut at an angle resembling the slanted tip of a quill pen. French separated the two senses: 'plume' (feather, from Latin 'pluma') took over the feather meaning, while 'plume' also came to mean 'pen' in the literary sense (as in 'nom de plume'). Spanish uses 'pluma' for both pen and feather.
Figurative Development
German and other Germanic languages preserve a parallel semantic development with their own native word. German 'Feder' means both 'feather' and 'pen' (specifically a nib pen or fountain pen), and 'Federkiel' (feather-quill) is the word for a quill pen. This confirms that the metaphorical transfer from feather to writing instrument occurred independently in multiple European language traditions.
The transition from quill to metal pen was gradual. Metal nibs existed in antiquity — bronze pens have been found in Roman archaeological sites — but they were uncommon. The steel-nib dip pen became practical in the 1820s and 1830s, with manufacturers like Joseph Gillott, Josiah Mason, and John Mitchell mass-producing steel nibs in Birmingham, England. The fountain pen, with its internal ink reservoir, was patented in various forms throughout the nineteenth century, reaching commercial viability with Lewis Waterman's design in 1884. The ballpoint pen, invented by Laszlo Biro in 1938 and mass-produced after World War II, finally ended the quill pen's multi-century reign. Through all of these technological revolutions, the word 'pen' persisted, its feathery origin invisible to most users.
The word has generated an important set of derivatives and compounds. 'Penknife' (a small folding knife) was originally the knife used to cut and sharpen quill pens. 'Penmanship' (handwriting skill) dates from the seventeenth century. 'Pen name' (a pseudonym used by an author) is a calque of French 'nom de plume,' though the French phrase was actually coined in English — native French speakers say 'nom de guerre' or 'pseudonyme.' 'Pen pal' dates from the 1930s.
Latin Roots
The proverb 'the pen is mightier than the sword' was coined by Edward Bulwer-Lytton in his 1839 play 'Richelieu,' though the idea that writing has more power than force is ancient. The Latin phrase 'calamus gladio fortior' (the reed pen is stronger than the sword) expresses the same thought. In Islam, the pen (al-qalam) has special significance: the Quran's 68th surah is titled 'Al-Qalam' (The Pen), and tradition holds that the first thing God created was the pen, with which He inscribed destiny.
The writing 'pen' (from Latin 'penna,' feather) is entirely unrelated to the enclosure 'pen' (from Old English 'penn,' an enclosure for animals), despite their identical modern spelling and pronunciation. The two words are homonyms with completely different etymological histories — one from Romance, one from Germanic.