Webster's Revised Unabridged Dictionary · 1913

Blackguard

Blackguard (blag"gard) , noun

[Black + guard.]

1.
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. [Obsolete]
A lousy slave, that... rode with the black guard in the duke's carriage, 'mongst spits and dripping pans. — Webster (1612)
2.
The criminals and vagrants or vagabonds of a town or community, collectively. [Obsolete]
3.
A person of stained or low character, esp. one who uses scurrilous language, or treats others with foul abuse; a scoundrel; a rough.
A man whose manners and sentiments are decidedly below those of his class deserves to be called a blackguard. — Macaulay
4.
A vagrant; a bootblack; a gamin. [Obsolete]

Blackguard , transitive verb

To revile or abuse in scurrilous language. — Southey

Blackguard , adjective

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