under cover—
1. under a shelter protecting from rain, wind, etc:
- Monsieur had better come under cover. The “Bosches” are still firing this way.
2. (of police or secret service agents) operating in secret:
- He worked under cover … in some of the most important theaters of the Cold War.
3. (dated or formal) in an envelope:
- The letter was passed to the Attorney General who sent a copy of the letter under cover asking for comment.
Cf.: under plain cover—in an envelope which has no indication of the firm, the contents, etc.:
- Credit card statements do not show our company name and all products are shipped under plain cover.
Note: The expression does not correlate in meaning with the phrase under wraps—used of smth. kept secret:
- The investigation apparently cleared the officers of any wrongdoing but the details are still under wraps.
undercover —(attrib.)
1. situated under the roof; sheltered from the elements:
- Undercover storage for vehicles is available for either short or long term requirements.
2. (of methods, agents, etc.) underground; secret:
- The menace of recruitment to undercover prostitution on a large scale cannot be ignored.