early 15c., "place of punishment for offenses against the church," from M.L. penitentiaria
, from fem. of penitentiarius
(adj.) "of penance," from L. paenitentia
"penitence" (see penitence
). Meaning "house of correction" first found 1806 (originally an asylum for prostitutes). Slang shortening pen
is attested from 1884.