blackguard: noun

black·​guard | \ ˈbla-gərd , -ˌgärd; ˈblak-ˌgärd \
1. The lowest servant in a household charged with pots, pans, and other kitchen equipment.
2. A scoundrel; an unprincipled contemptible person; an untrustworthy person.
3. A man who uses foul language in front of a woman, typically a woman of high standing in society.
4. A man who behaves in a dishonourable or contemptible way.

blackguard: verb

To revile or abuse in scurrilous language.
Abuse or disparage (someone) scurrilously.
To act like a blackguard; to be a scoundrel.

Etymology: From black + guard, thought to have referred originally to the scullions and lower menials of a court, or of a nobleman's household, who wore black liveries or blacked shoes and boots, or were often stained with soot.