horny comes from horn + -y; horn descends from Old English horn ('horn'), from Proto-Germanic *hurną, from Proto-Indo-European *ḱr̥no-, from *ḱer- ('head, horn'). The slang sense 'aroused' arose in the 18th century. 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 "made of or resembling horn; also slang for sexually aroused", so the historical development ends in a clearly identifiable modern dictionary use. The same base also appears in related forms such as horn and horned, and in derivatives such as horniness. Its semantic neighborhood includes aroused, lustful, randy, and lecherous, which helps locate the modern sense without adding a separate etymological claim. The named historical forms include Old English horn, Proto-Germanic *hurną, and Proto-Indo-European *ḱer-.