The word penthouse is not connected with ‘five’ (as with most words beginning pent-), and neither is it derived from house. It entered English in the Middle Ages as pentis, from Old French apentis, which was based on Latin appendicium ‘appendage’, from appendere ‘hang on’ (the source of appendage and appendix). The form changed in the 16th century because it was associated with French pente ‘slope’ and the English word house. At this time a penthouse was an outhouse or lean-to with a sloping roof; it did not take on its modern sense until the end of the 19th century.