late 14c., "made of horn," from horn (n.) + -y (2). From 1690s as "callous, resembling horn." The colloquial meaning "lustful, sexually aroused," was in use certainly by 1889, perhaps as early as 1863; it probably derives from the late 18c. slang expression to have the horn, suggestive of male sexual excitement (but eventually applied to women as well); see horn (n.). As a noun it once also was a popular name for a domestic cow. For an adjective in the original sense of the word, hornish (1630s) and horn-like (1570s) are available.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).