This is the 12726th most frequent Polish word.
nieprawdą
"Nieprawdą" means "not true" or "falsehood" in English.
In this sentence, 'nieprawdą' is a form of the noun 'nieprawda,' used in its instrumental case to indicate disagreement with the factuality of a statement.
To, co powiedział, jest nieprawdą.
What he said is untrue.
Here, 'nieprawdą' serves as a predicate noun, emphasizing the falseness of the preceding statement.
Nieprawdą jest, że nie lubię kawy.
It is untrue that I do not like coffee.
In this example, 'nieprawdą' is used as an object in its instrumental case to refer to something deemed untrue, expressing opposition to it.