This is the 17651st most frequent Polish word.
zimna
"Zimna" means "cold" in English.
The word 'zimna' is the feminine form of the adjective 'zimny' and is used here to describe the noun 'woda', which is feminine.
Nie lubię zimnej wody.
I don't like cold water.
Here 'zimne' is the neuter form of 'zimny', corresponding to the neuter noun 'powietrze'.
To było zimne poranne powietrze.
It was cold morning air.
In this sentence 'zimny' is the masculine form, as it describes the masculine noun 'dzień'.