The idiom "on the sly" originated from the word "sly," which means cunning or crafty. The phrase refers to doing something secretly or discreetly, without others' knowledge or approval. It is used to convey a sense of stealthiness or furtiveness in one's actions, often implying that the activity being discussed is meant to be concealed or kept hidden from others' awareness.

英語での「on the sly」の定義と意味

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