undercover:1

adjective
un*der*cov*er \ ˌən-dər-ˈkə-vər \
1920
  • acting or executed in secret; specifically : employed or engaged in spying or secret investigation

undercover:2

noun
undercover \
1962
  • a person engaged in undercover activity : spy

cover:2

noun
cover \
14th century{ds||1||}
  • something that protects, shelters, or guards: such as
  • natural shelter for an animal; also : the factors that provide such shelter
  • a position or situation affording protection from enemy fire

under cover of anonymity

idiom
under cover of anonymity \
  • with the understanding that one's name will not be revealed

under separate cover

idiom
under separate cover \
  • in a separate envelope

under (the) cover of darkness/night

idiom
under (the) cover of darkness/night \
  • in the dark : at night
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