This is the 1368th most frequent Dutch word.
stiekem
Secretly; covertly; sneakily.
In this sentence, 'stiekem' is used to describe the act of taking the cookie secretly, without wanting others to notice.
Ik nam stiekem een koekje uit de schaal.
I secretly took a cookie from the dish.
Here, 'stiekem' is used to illustrate the confidential manner of sharing the secret, done quietly and privately.
Hij vertelde stiekem zijn geheim aan zijn beste vriend.
He secretly told his secret to his best friend.
In this usage, 'stiekem' refers to the internal, private knowledge the subject possesses without making it obvious to others.