A person of stained or low character, esp. one who uses scurrilous language, or treats others with foul abuse; a scoundrel; a rough.

To revile or abuse in scurrilous language.

Scurrilous; abusive; low; worthless; vicious; as, blackguard language.

use foul or abusive language towards; "The actress abused the policeman who gave her a parking ticket"; "The angry mother shouted at the teacher"

an evil villain of the first order, equivalent in power to the righteous paladin, but devoted to the powers of darkness

use obscene language

(pronounced 'blaggard') scoundrel or villain

In Dungeons & Dragons, Blackguard is a base character class in older versions of the game, and a prestige class as of edition 3.5. A blackguard is the antonym of a paladin and is an evil warrior who receives a variety of magical powers from binding pacts with Fiends or evil gods.