The word ribbed can be described as having raised parallel lines on the surface. An antonym for ribbed would be smooth, which means having an even, regular surface without any roughness or projections. Another antonym for ribbed could be flat, which means having a level surface without any curves or bumps. The opposite of ribbed can also be curved, which means having a surface that is gently rounded or bent. Alternatively, the word plain can be used as an antonym for ribbed, which means being without any special or distinctive features. In summary, smooth, flat, curved, and plain are all suitable antonyms for the word ribbed.