Source : Webster's Revised Unabridged Dictionary (1913)

Horehound \Hore"hound`\, n. [OE. horehune, AS. h[=a]rhune;
   h[=a]r hoar, gray + hune horehound; cf. L. cunila a species
   of organum, Gr. ?, Skr. kn?y to smell.] (Bot.)
   A plant of the genus {Marrubium} ({M. vulgare}), which has a
   bitter taste, and is a weak tonic, used as a household remedy
   for colds, coughing, etc. [Written also {hoarhound}.]

   {Fetid horehound}, or {Black horehound}, a disagreeable plant
      resembling horehound ({Ballota nigra}).

   {Water horehound}, a species of the genus {Lycopus},
      resembling mint, but not aromatic.

Source : WordNet®

black horehound
     n : ill-smelling European herb with rugose leaves and whorls of
         dark purple flowers [syn: {black archangel}, {fetid
         horehound}, {stinking horehound}, {Ballota nigra}]