helly comes from hell (Old English hell, from Proto-Germanic *haljō) + the adjectival suffix -y; a rare and dialectal formation parallel to hellish. 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 "hellish or infernal in character (rare and dialectal)", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as hell and hellish. Its semantic neighborhood includes hellish, infernal, fiendish, and diabolical, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English hell and Proto-Germanic *haljō.