clandestine
adj.adj. done in secret, often because it is illegal or not allowed. You use this to describe things like hidden meetings or private plans.
adj. kept secret or done secretively, especially because illicit. Typically describes activities, meetings, or relationships rather than physical objects.
The two spies had a clandestine meeting in the park.
The group held clandestine meetings in a basement to plan their protest without the police finding out.
Historians recently uncovered documents detailing the clandestine operations carried out by the resistance during the final months of the occupation.
From Latin clandestīnus (“secret, concealed”); compare French clandestin.
Often used attributively before a noun; frequently collocates with 'meeting', 'operation', or 'affair'.