The word "more ribbed" refers to something that has more raised lines or ridges on its surface. Its antonyms are smooth, flat, even, and plain. Smooth refers to a surface that is free of bumps, lumps, or roughness. Flat means without curves, dips, or rises. Even indicates that a surface is uniform and level, without any lumps or depressions. Plain describes a surface that is unadorned or unembellished, without any patterns, textures, or designs. These antonyms are useful in describing the opposite of something that is "more ribbed" and can be used in various contexts such as clothing, skin, or building materials.