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1. Protected by a shelter, as in It began to pour but fortunately we were under cover. [c. 1400]
2. under cover of. Also, under the cover of. Hidden or protected by, as in They sneaked out under cover of darkness, or, as it was put in a sermon in 1751: "Presumption which loves to conceal itself under the cover of humility" (John Jortin, Sermons on Different Subjects).
1 pretending to be somebody else in order to do something secretly: a police officer working under cover
2 under a structure that gives protection from the weather: We’d better get under cover or we’ll get very wet in this rain.
1. In an enclosure for mailing.
2. Being hidden or protected, as by darkness.