'卧底' (wò dǐ) literally means 'lying at the bottom' — 卧 (wò) means 'to lie down' or 'to recline', and 底 (dǐ) means 'bottom' or 'base'. Together, they metaphorically describe someone who lies hidden deep within an opposing group, like a spy embedded undercover. This term is almost exclusively used in contexts of law enforcement, intelligence, or organized crime — for example, a police officer pretending to be a criminal to gather evidence from inside.

The word carries strong connotations of secrecy, risk, and deception, and is often found in crime dramas, news reports about sting operations, or discussions of counterintelligence. It is not used for everyday concealment (e.g., hiding keys), nor for legitimate, open roles like informants who cooperate publicly. While vivid and widely understood, 卧底 is slightly informal and dramatic; formal documents may prefer terms like ' undercover agent' (便衣警察) or 'deep-cover operative' (潜伏人员).