If you do something subreptitiously, you do it secretly or sneakily, so no one will notice.
Doing something subreptitiously is usually not a good thing because it means you are being sneaky or dishonest.
Examples:
He subreptitiously took a cookie from the jar.
She subreptitiously slipped the note into his bag.
The cat subreptitiously snuck up on the bird.
The squirrel subreptitiously stole the nuts from the bird feeder.
The spy subreptitiously gathered information without anyone knowing.
The ninja subreptitiously moved through the shadows.