secretThe spies had a secret meeting.
covertThe government was accused of covert military operations.
clandestineformalHe arranged a clandestine meeting between his client and the candidate.
confidentialAll the information you give us will be treated as strictly confidential.
innermostShe never told anyone her innermost thoughts.
privateThey kept their views on the subject private.
furtiveHe kept giving her furtive glances.
surreptitiousShe took a surreptitious look at her watch.
under wrapsHe is doing some research for a new book which is under wraps at the moment.