surreptitious

The word "surreptitious" describes something done in a secret, stealthy, or clandestine way, usually because it would not be approved of if it were done openly. The mental image is of someone glancing around nervously to ensure they aren't being watched while they sneak a look at something or perform a private action. It is a sophisticated, formal word that carries a hint of guilt or illicit intent. While "secret" simply means hidden, "surreptitious" emphasizes the act of avoiding detection through sly behavior. It is frequently used with nouns like "glance," "look," or "movement."
Examples
She took a surreptitious bite of the cake while her mother wasn't looking.
She took a surreptitious bite of the cake while her mother wasn't looking.
The spy made a surreptitious exit from the building through the back door.
The spy made a surreptitious exit from the building through the back door.
Their surreptitious meetings in the park were eventually discovered by the authorities.
Their surreptitious meetings in the park were eventually discovered by the authorities.