>>86024>>86013Looked it up because I was curious, and it kind of ties into that other thread about how using French words makes you sound sophisticated. Mansion was originally a French term used for a home/permanent abode, but sometime in the mid 14th century English lords started using it to refer to their primary residence. So eventually the previously French word for any home became an English term used for big houses.
Also it appears that it's still somewhat common in the UK to refer to upscale apartment buildings as mansions, probably where the Japanese borrowed the term from.