Origins
The word 'mansion' is a study in semantic inflation. It entered English meaning simply 'a place to stay' and gradually swelled in meaning until it signified one of the grandest forms of residential architecture. The story of how a humble Latin word for 'remaining' became synonymous with wealth and splendor is a social history as much as a linguistic one.
The Latin source is 'mānsiō' (genitive 'mānsiōnis'), a noun formed from the verb 'manēre' (to remain, to stay, to endure). In Roman usage, a 'mansio' was a stopping place or station along a road — an inn or rest house where travelers could stay for the night. The Roman road network was dotted with 'mansiones,' maintained by the state, typically a day's journey apart. The word had no connotation of grandeur; it simply meant 'a place where you remain.'
The word passed through Vulgar Latin into the Romance languages, where it evolved in different directions. In French, 'mansion' became 'maison,' which today simply means 'house' — any house, from a cottage to a château. The French word preserved the unpretentious original: a maison is where you live, nothing more. Spanish took 'mansio' in a different direction: 'mesón' means an inn or tavern, preserving the original Roman sense of a traveler's stopping place.
French Influence
English borrowed 'mansion' from Old French in the fourteenth century, and for several centuries it meant simply 'a dwelling place,' 'an abode,' or 'a separate apartment within a larger building.' The most famous early English use is in the King James Bible (1611), John 14:2: 'In my Father's house are many mansions.' The translators were rendering Latin 'mansiones' (from the Vulgate) and Greek 'μοναί' (monai, dwelling places). Jesus was not promising his disciples palaces; he was promising them rooms — places to stay in his Father's house. The modern association of 'mansion' with wealth and grandeur makes the verse feel more opulent than it was originally intended.
The semantic narrowing to 'a large, impressive house' occurred gradually during the seventeenth and eighteenth centuries, driven by social usage. As English landowners built increasingly grand country houses, 'mansion' became the term of choice for the largest and most impressive of these residences. By the nineteenth century, the original sense of 'any dwelling' was obsolete, and 'mansion' meant exclusively a large, luxurious house.
The Latin verb 'manēre' produced a substantial English word family through various paths. 'Remain' (from re- + manēre, to stay back) came through French. 'Permanent' (from per- + manēre, to stay through, to endure) came through Latin. 'Manor' (from Old French 'manoir,' a dwelling, from Latin 'manēre') is a near-synonym that followed a parallel path. 'Menagerie' (originally a household or farm, from Old French 'mesnage,' a household, from Vulgar Latin *mansionāticum) acquired its modern meaning of a collection of wild animals because noble households kept exotic animals on their estates.
Latin Roots
The contrast between English 'mansion' and French 'maison' is one of the most instructive pairs in comparative etymology. The same Latin word, borrowed into two languages at similar times, evolved in radically different directions. French kept it democratic; English made it aristocratic. A French 'maison' can be a shack; an English 'mansion' must be grand. The divergence reflects not a linguistic process but a social one — the word inflated as the buildings it described grew larger and more pretentious.