Hilly, a.
1. Abounding with hills; uneven in surface; as, a hilly country. "Hilly steep." Dryden.
2. Lofty; as, hilly empire. [Obs.] Beau. & Fl.
Shades of meaning
Fernald lists hilly only as an antonym, under horizontal.
He lists it without comment; the entries above explain its neighbors.
Synonyms
none given