pen¹ / pen/ • n. 1. an instrument for writing or drawing with ink, typically consisting of a metal nib or ball, or a nylon tip, fitted into a metal or plastic holder. ∎  (the pen) the occupation or practice of writing: she was forced to support herself not only by the pen, but as a secret agent. ∎  an electronic penlike device used in conjunction with a writing surface to enter commands or data into a computer.2. Zool. the tapering cartilaginous internal shell of a squid.• v. (penned, pen·ning) [tr.] write or compose: he had not penned a line to Lizzie in three years.PHRASES: put (or set) pen to paper write or begin to write something.pen² • n. a small enclosure in which sheep, pigs, cattle, or other domestic animals are kept. ∎  a number of animals in or sufficient to fill such an enclosure: a pen of young horses. ∎  any small enclosure in which someone or something can be confined. ∎  a covered dock for a submarine or other warship.• v. (penned, pen·ning) [tr.] put or keep (an animal) in a pen: it was the practice to pen the sheep for clipping. ∎  (pen someone up/in) confine someone in a restricted space: they had been penned up day and night in the house.pen³ • n. a female swan.pen⁴ • n. inf. short for penitentiary (sense 1).

pen2 instrument, orig. quill, for writing XIII; (dial.) feather XIV. ME. penne — (O)F. — L. penna feather, pl. pinions, wings, in late L. pen; see FEATHER.
Hence pen vb.2 XV. penknife XV. orig. one used for mending quill pens.

pen the pen is mightier than the sword proverbial saying, late 16th century, meaning that written words may often have more lasting force than military strength; the same idea is found in the writings of the Roman orator and statesman Cicero (106–43 bc), ‘arms give way to persuasion.’
pen-name an assumed name used by a writer instead of their real name.

PEN • abbr. International Association of Poets, Playwrights, Editors, Essayists, and Novelists.

pen1 enclosure for domestic animals. XIV. presumably repr. OE. penn, evidenced only in designations of local features of uncertain meaning; implied in OE. onpennad ‘unpenned’, opened.
So pen vb.1 enclose, confine. XII.