From blackguard -- either concerning street urchins who would blacken clients' shoes, or from the house servants who worked in the dark quarters of the house: a person whose conduct or character is disgraceful, disgusting, morally unfit; a thief with evil intent; a contemptible scoundrel; a term of abuse; a foulmouthed person
I won't have that blaggard take one step in my quarters!
A person who manages to get away with not doing something by way of talking their way out of it, or by just going on and on and on until someone else does it or you just give up and do it yourself or by the time they've finished it doesn't need doing any more.
Man, I asked Mark to go and serve that customer and he just started fucking talking about something else and wouldn't shut up. By the time he'd finished Paul had stepped in served them. What a blaggard!
from the British slang blag which means to steal or rob usually by force, a blagger is the robber who usually robs by force, the word blaggard is a variance on these two concepts a slang on the slang if you will meaning a rogue-ish bastardly person, a con.