Clandestine (adjective)

Kept or done secret or in secret, especially because illicit or of questionable nature.

Origin:

Late 16th century: from french clandestin, from latin clandestīnus, from clam 'secretly' + destinare 'destine'.

Examples:

1. The couple were caught in a clandestine affair.
2. He took a clandestine approach to his business dealings.
3. The secret group conducted their meetings in a clandestine manner.
4. The police found evidence of a clandestine drug operation.
5. She had a clandestine meeting with her lover.