The etymology for the term clandestine (klan-ˈdes-tən) is from the 1560’s. It is Latin from clandestinus “secret, hidden,”and clam meaning “secretly,” combined with intestinus Latin for “internal.” It is an adjective denoting something executed with secrecy or concealment, especially for purposes of subversion or deception; private or surreptitious.
Example: Their clandestine meetings went undiscovered for two years.
Underused word in my opinion. :)