This is the 5211th most frequent Korean word.
단단히
"Firmly" or "securely."
Here, '단단히' is used to describe the firmness or resoluteness of her decision.
그녀는 단단히 결심했다.
She firmly made up her mind.
In this sentence, '단단히' indicates securely or tightly in the context of closing a door.
문을 단단히 닫아라.
Close the door tightly.
'단단히' is applied here to emphasize the tightness or security of tying shoe laces.