In a dishonorable, morally corrupt, or contemptible manner; in a way that shows lack of integrity or dignity.
From base (of low moral character, from Old French bas) + -ly (adverb-forming suffix). This adverb has been used since Middle English to describe shameful actions.
When Shakespeare has a character act basely—betraying a friend or lying shamefully—the word choice itself makes readers feel the moral weight of the action, not just the facts of what happened.
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