A Jacuzzi is a big indoor bath that throws hot water at you in strong little streams, so the water keeps pushing and rubbing your body instead of sitting still. People sit in it to ease tired muscles, to warm up, or because it feels a bit luxurious — hotels, gyms, and some homes have one. The name began as a company name, from the Jacuzzi family who made these baths; then English speakers started using Jacuzzi for any bath like that, even ones another company built. You will still often see a capital J, like a brand, but in everyday talk it behaves like an ordinary noun: “the jacuzzi.” It is not a normal bathtub, and it is not an outdoor hot-spring pool. Some people also stretch it into a verb, “to jacuzzi,” meaning to sit in one; that use is looser and less common.