1.characterized by, done in, or executed with secrecy or concealment, especially for purposes of subversion or deception; private or surreptitious:
Their clandestine meetings went undiscovered for two years.
Origin of clandestine
1560-1570 Latin clandestīnus, equivalent to *clande, *clamde, variant of clam secretly (with -de adv. particle) + -stīnus, probably after intestīnusinternal;