From Middle English pen, penne, from Old English penn, from Proto-Germanic *pennō, *pannijō, from Proto-Indo-European *bend-. Related to pin.
Sense “prison” originally figurative extension to “enclosure for persons” (1845), later influenced by penitentiary, being analyzed as an abbreviation (1884).
, derived from penn (see above). Akin to Low German pennen.
From Middle English penne, from Anglo-Norman penne, from Old French penne, from Latin penna, from Proto-Indo-European *péth₂r̥, from *peth₂- (from which petition). Proto-Indo-European base also root of *petra-, from which Ancient Greek πτερόν (whence pterodactyl), Sanskrit पत्रम्, Old Church Slavonic перо, Old Norse fjǫðr, Old English feðer (Modern English feather); note the /p/ → /f/ Germanic sound change.