Here are some synonyms for surreptitious:
* Secret
* Covert
* Clandestine
* Underhanded
* Sneaky
* Furtive
Example Sentences:
* The spy made a surreptitious entry into the building, avoiding the guards.
* She took a surreptitious glance at her watch, hoping no one noticed.
* The thief made a surreptitious grab for the wallet.