horehound
Concise Oxford English Dictionary © 2008 Oxford University Press:
horehound /ˈhɔːhaʊnd/ (also hoarhound)
▶noun a strong-smelling hairy plant of the mint family, traditionally used as a medicinal herb. [Marrubium vulgare (white horehound) and Ballota nigra (black horehound).]
'horehound' also found in these Oxford entries:

