Is it ensure, insure, or assure?
early 15c., "place of punishment for offenses against the church," from Medieval Latin penitentiaria, from fem. of penitentiarius (adj.) "of penance," from Latin paenitentia "penitence" (see penitence). Meaning "house of correction" (originally an asylum for prostitutes) is from 1806, short for penitentiary house (1776). Slang shortening pen is attested from 1884.