If you do something secretly, you do it without other people knowing about it.
Doing something secretly can make people feel sneaky or like they are doing something they shouldn't be doing.
Examples:
She secretly ate the cookies when no one was looking.
He secretly bought a present for his mom.
The cat secretly snuck into the room.
I secretly stayed up past my bedtime.
The squirrel secretly took the nuts and buried them in the garden.
The dog secretly buried the bone in the backyard.