appendicium

Penthouse derives from Anglo-French pentis, from Medieval Latin appendicium (an appendage), from Latin appendere (to hang upon). A pentis was a structure attached to the wall of a larger building — a lean-to roof, a shed, an awning. It was subordinate architecture, the thing that hung off the real building. The word had no association with height, luxury, or exclusivity. It was the architectural afterthought.

The 'house' in penthouse is a folk etymology. The original word was pentis or pentice. English speakers heard the unfamiliar French word, associated it with 'house,' and respelled it accordingly. The transformation was complete by the sixteenth century. But even after gaining 'house,' the word still meant a lean-to or a small attached structure — typically a roof over a doorway or a covered passage attached to a building's side.

The modern meaning appeared in the early twentieth century, when New York City building codes required that mechanical equipment on rooftops be enclosed in structures. These rooftop enclosures were called penthouses — a revival of the architectural term for an appendage attached to a larger building. Developers realized that the space surrounding these rooftop structures could be converted into luxury apartments. The penthouse moved from the base of the building to the top, and from the cheapest to the most expensive space.

The penthouse apartment is now the most prestigious unit in any residential building. Penthouses are defined by height, views, and exclusivity. The word that originally named a lean-to shed now names apartments that sell for hundreds of millions of dollars. The progression from humble appendage to ultimate luxury is one of the most extreme semantic reversals in English.