Horn  n. 1 a hard outgrowth, often curved and pointed, on the head of esp. Hoofed animals. B each of two branched appendages on the head of (esp. Male) deer. C hornlike projection on animals, e.g. A snail's tentacle. 2 substance of which horns are made. 3 mus. A = *french horn. B wind instrument played by lip vibration, orig. Made of horn, now usu. Of brass. 4 instrument sounding a warning. 5 receptacle or instrument made of horn. 6 horn-shaped projection. 7 extremity of the moon or other crescent. 8 arm of a river etc.  horn in slang intrude, interfere.  horned adj. Hornist n. (in sense 3 of n.). [old english]