In a deceitful, cunning, or tricky manner; with dishonest cleverness.
Adverbial form of 'guileful' created by adding '-ly' (Old English 'lice' meaning 'in the manner of'). The formation follows standard English patterns for converting adjectives to adverbs.
This word is basically the adverb version of Machiavellian behavior—it describes acting with purposeful deception and calculated cunning, showing that English evolved precise language for describing manipulation.
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