In the manner of a blackguard; dishonorable, unscrupulous, or befitting a rogue.
From 'blackguard' + '-ly' (adjective/adverb suffix). An older English formation describing behavior fitting a contemptible person.
Blackguardly sneers and blackguardly tricks populate Victorian novels—it's the perfect word for describing the mustache-twirling villainy that 19th-century literature loved to exaggerate.
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