1. A rude or unscrupulous person; a scoundrel.
2. A person who uses foul or abusive language.
adjective: Scurrilous; abusive; low; worthless; vicious; as, "blackguard language."
transitive verb: To revile or abuse in scurrilous language.
Douglas was not a saint, though, so his behaviour and attitude were, as he wrote, 'neither better nor worse than my contemporaries -- that is to say, [I became] a finished young blackguard, ripe for any kind of wickedness'.
--Douglas Murray, [?]Bosie: A Biography of Lord Alfred Douglas
The years, as time went on, imparted to him that peculiar majesty that white-haired blackguards, successful (and unpunished) criminals, seem generally to possess.
--Saul David, [?]Prince of Pleasure
Monroe wondered, but did not ask, what could have driven a young lady of such fine bearing and aristocratic attraction to leave home at a tender age and follow the fortunes of a blackguard like Reynolds.
--William Safire, [?]Scandalmonger
When we want to talk friendly with him, he will not listen to us, and from beginning to end his talk is blackguard.
--Tecumseh, quoted in [?]Tecumseh: A Life, by John Sugden
Blackguard is from black + guard. The term originally referred to the lowest kitchen servants of a court or of a nobleman's household. They had charge of pots and pans and kitchen other utensils, and rode in wagons conveying these during journeys from one residence to another. Being dirtied by this task, they were jocularly called the "black guard."