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undercover

Undercover means secret or disguised. A police department might send undercover officers dressed as clowns to investigate a corrupt circus.

When this adjective was originally used in the mid-19th century, it meant "sheltered beneath something," but after a century or so, it was routinely used to mean "covert or clandestine." If you go undercover, you're doing some kind of secret work, usually inside an organization or group. If the other chess club members want you to be an undercover spy inside the French club, you'll have to brush up on your verb tenses.

Definitions of undercover
  1. adjective
    conducted with or marked by hidden aims or methods
    “an undercover investigation”
    synonyms: clandestine, cloak-and-dagger, hole-and-corner, hugger-mugger, hush-hush, secret, surreptitious, underground
    covert
    secret or hidden; not openly practiced or engaged in or shown or avowed
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