noun a rude or unscrupulous person who behaves in a dishonourable or contemptible way.
One of the characteristics of Hiberno (Irish) English is that there are many words commonly used in speech that are archaic in standard British English.
A good example is the word blackguard, meaning a person "of low or worthless character." This probably originated in disparaging references to the lower levels of the military and/or the livery worn by bootblacks and other low status servants.
Citations
This footnote to an 1816 edition the plays of Ben Johnson suggests that the word was around at the time of Shakespeare.
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| The Works of Ben Jonson, annotated in 1816 by William Gifford |
Crime
By the late 18th Century blackguard had become more directly associated with criminality. This association continued but the word had faded into obscurity in British English by the 1950s.
Blackguard has remained in common use in Ireland (where there is plenty of use for it, as the joke goes). It also remains popular amongst the Irish diaspora.