Cunning and deceitful in a clever way; doing things secretly or dishonestly while trying to hide your intentions.
From Old Norse 'slœgr' meaning 'able to strike' or 'clever.' The word came to English through Scandinavian influence and evolved to mean cunning or deceitful.
The original Norse meaning of 'sly' was about being skilled at striking—it was almost admiring. But as it evolved in English, it took on a moral judgment, becoming associated with dishonesty rather than just cleverness, showing how languages can change a word's moral tone.
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