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).

pen (n.2)

"small enclosure for domestic animals," Old English penn, penne, a word of uncertain origin, perhaps related to Old English pinn "pin, peg" (see pin (n.)) on the notion of a bolted gate or else "structure made of pointed stakes."

pen (v.1)

"to write, compose and commit to paper," late 15c., from pen (n.1). Related: Penned; penning.

pen (v.2)

"to confine or enclose in a pen," c. 1200, pennen, from Old English *pennian (only in compounds), from the source of pen (n.2). Related: Penned; penning.

pen (n.3)

slang, "prison," 1884, shortening of penitentiary; earlier use (1845) probably is a figurative extension of pen (n.2) "enclosure for animals."

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Definitions of pen from WordNet
pen (n.)
a writing implement with a point from which ink flows;
pen (n.)
an enclosure for confining livestock;
pen (n.)
a portable enclosure in which babies may be left to play;
Synonyms: playpen
pen (n.)
a correctional institution for those convicted of major crimes;
Synonyms: penitentiary
pen (n.)
female swan;
pen (v.)
produce a literary work;
Synonyms: write / compose / indite