Nipple: from body part to threaded connector

To most people, a nipple is a small projection on the human body, specifically the raised point at the center of the breast or chest. The word comes from Old English and remains purely anatomical in everyday speech. But step onto a plumbing job site and the term shifts entirely to describe a piece of metal or plastic hardware.

In plumbing, a nipple is a short length of pipe, usually brass, steel, or PVC, with threads cut on both ends. Its sole purpose is to connect two female-threaded fittings, such as elbows, tees, or valves. A typical nipple might be 1 to 4 inches long and range from 1/4 inch to 2 inches in diameter. The plumber screws it into one fitting, then screws the second fitting onto the other end. It bridges the gap where a standard pipe would be too long or where a more compact assembly is needed.

Nipples come in various materials and thread types. Steel nipples are common in high-pressure or high-temperature applications. Brass resists corrosion better in water systems. PVC nipples suit drain lines and cold-water circuits where plastic is acceptable. Threads may be national pipe thread (NPT), the standard in North America, or other specifications depending on the fitting it must join.

The metaphorical link between the anatomical nipple and the plumbing nipple likely stems from the resemblance: both are small, raised, protrusive features. In older plumbing literature and among experienced tradespeople, the term feels natural because the shape and function align with a miniature projection. The word followed plumbing into common technical use sometime in the 19th or early 20th century, when standardized pipe fittings became prevalent, though the exact date of adoption is not recorded in standard plumbing histories.