Antonyms of adj hilly

1 sense of hilly

Sense 1
cragged, craggy, hilly, mountainous -- (having hills and crags; "hilly terrain")

INDIRECT (VIA rough) -> smooth -- (having a surface free from roughness or bumps or ridges or irregularities; "smooth skin"; "a smooth tabletop"; "smooth fabric"; "a smooth road"; "water as smooth as a mirror")

Similarity of adj hilly

1 sense of hilly

Sense 1
cragged, craggy, hilly, mountainous -- (having hills and crags; "hilly terrain")
       => rough (vs. smooth), unsmooth -- (having or caused by an irregular surface; "trees with rough bark"; "rough ground"; "rough skin"; "rough blankets"; "his unsmooth face")