hilly comes from hill (Old English hyll, from Proto-Germanic *hulliz) + the adjectival suffix -y. Morphologically, the ending is part of the etymology: the suffix is attached to the older base named in the history, producing a later English derivative. In present English, the adjective sense is "having many hills; abounding in slopes", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as hill and hillock, and in derivatives such as hilliness. Its semantic neighborhood includes undulating, rolling, mountainous, and steep, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English hyll, Proto-Germanic *hulliz, and Proto-Indo-European *kelH-.