(1):

(n.) A vagrant; a bootblack; a gamin.

(2):

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

(3):

(n.) The scullions and lower menials of a court, or of a nobleman's household, who, in a removal from one residence to another, had charge of the kitchen utensils, and being smutted by them, were jocularly called the "black guard"; also, the servants and hangers-on of an army.

(4):

(v. t.) To revile or abuse in scurrilous language.

(5):

(a.) Scurrilous; abusive; low; worthless; vicious; as, blackguard language.

(6):

(n.) The criminals and vagrants or vagabonds of a town or community, collectively.