harts·horn \ˈ ̷ ̷ˌ ̷ ̷\noun (-s) Etymology: Middle English hertes horn, from Old English heortes horn, from heortes (gen. of heort hart) + horn 1.: a hart's horn or antler 2.archaic a.: ammonia water b.: ammonium carbonate c 3.orhartshorn plantain: buckhorn 3b(1)