undercover

秘密的，秘密从事的；从事间谍活动的英释中释adjUndercover work involves secretly obtaining information for the government or the police

1 undercover agent,卧底；警方密探

1 ...an undercover operation designed to catch drug smugglers..…一次为抓捕毒品走私贩而策划的秘密行动。

2 ...undercover FBI agents..…联邦调查局的密探。