A "reverse bathtub" has a couple of distinct meanings depending on the context. In design, the Reverse bathtub by Cesana is a minimalist and versatile tub with a unique shower bathscreen composed of two independent doors and a sliding teak shelf/seat, functioning both as a bathtub and a shower enclosure. Its design includes practical features like the shower door handle doubling as a towel rail and the sliding seat as a shelf for toiletries, emphasizing ease of use and maintenance.

In a different architectural or construction context, a "reverse bathtub" refers to a waterproofing design measure meant to keep water out of a building's basement and lower floors. This involves sealing techniques to prevent water intrusion, essentially acting as a protective barrier against natural water penetration—hence the term "reverse" indicating the opposite function of a regular bathtub, which holds water in rather than keeping it out.

Additionally, in plumbing language, "reverse" can relate to the orientation or handedness of bathtub plumbing or drain placement but typically refers to reversible drain positions, allowing flexibility in installation.

Thus, "reverse bathtub" could mean either a specialized bathtub design with a unique door and seat mechanism, a construction water-sealing method around basements, or a plumbing term regarding drain orientation. The precise meaning depends on the specific industry or context being discussed.