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).
"partner, mate, chum," slang, 1680s, said to be from Romany (English Gypsy) pal "brother, comrade," a variant of continental Romany pral, plal, phral, which are probably from Sanskrit bhrata "brother" (from PIE root *bhrater- "brother"). Colloquial extended form palsy-walsy is attested from 1930. Pally (adj.) is attested by 1895.
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Harper, D. (n.d.). Etymology of pen-pal. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/pen-pal