This is the 14233rd most frequent German word.
heimlich
"Secret" or "concealed."
In this sentence, 'heimlich' is used as an adverb meaning 'secretly' or 'discreetly'.
Er schaute sich heimlich um.
He looked around secretly.
Here, 'heimlich' describes the way she planned the event, indicating that it was done quietly and unnoticed.
Sie plante das Fest heimlich.
She planned the celebration secretly.
In this example, 'heimlich' conveys a sense of stealth or silence in the cat's movements.