kept secret or done secretively, especially because illicit.
“she deserved better than these clandestine meetings”
In 1658, the English poet John Milton wrote of “clandestine Hostility covered over with the name of Peace.” Three and a half centuries later we use clandestine in much the same way. The word is often used as a synonym of secret and covert, and it is commonly applied to actions that involve secrecy maintained for an evil, illicit, or unauthorized purpose. It comes to us by way of Middle French from Latin clandestine, which is itself from the clam, meaning “secretly.”