To treat someone as a scoundrel or villain, or to behave like a blackguard (a dishonorable person).
From 'blackguard' (a rogue or contemptible person, origin unclear but possibly from 'black guard' referring to foot soldiers) + '-ize' (to cause to be or do).
Blackguardize is a delightfully Victorian verb that means to basically act like a dishonest jerk—it captures an entire attitude of 19th-century moral judgment in a single word.
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