Old English horn "horn of an animal," also "wind instrument" (originally made from animal horns), from Proto-Germanic *hurnaz (cognates: German Horn, Dutch horen, Gothic haurn), from PIE *ker- (1) "horn; head, uppermost part of the body," with derivatives refering to horned animals, horn-shaped objects and projecting parts (cognates: Greek karnon "horn," Latin cornu "horn," Sanskrit srngam "horn," Persian sar "head," Avestan sarah- "head," Greek koryphe "head," Latin cervus "deer," Welsh carw "deer"). Reference to car horns is first recorded 1901. Figurative senses of Latin cornu included "salient point, chief argument; wing, flank; power, courage, strength." Jazz slang sense of "trumpet" is by 1921. Meaning "telephone" is by 1945.

1690s, "to furnish with horns," from horn, n.. Earlier in figurative sense of "to cuckold" (1540s). Meaning "to push with the horns" (of cattle, buffalo, etc.) is from 1851, American English; phrase horn in "intrude" is by 1880, American English, originally cowboy slang.