This is the 12173rd most frequent Polish word.
martw
"Martw" in Polish means "dead" or "lifeless."
Here, 'martw' is used in an imperative form instructing someone not to worry.
Nie martw się, wszystko jest w porządku.
Don't worry, everything is fine.
Here, 'martwego' (a form of 'martwy') is used as an adjective describing someone as dead.
Wyglądał na martwego, ale wciąż oddychał.
He looked dead, but he was still breathing.
Here, 'martwo' is used as an adverb describing the manner of her voice, likening it to lifelessness.