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English definition of “underground”

underground

adjective, adverb (SECRET)    /ˌʌn.dəˈɡraʊnd/ US  /-dɚ-/
describes an activity that is secret and usually illegal: an underground newspaper/movement The Communist Party was forced (to go) underground, and its leaders went into hiding.Secrecy and privacyLegal and illegal
(Definition of underground adjectiveadverb (SECRET) from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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