This is the 13355th most frequent Polish word.
oszustwem
"Fraud" or "deception."
In this sentence, 'oszustwem' is used in the instrumental case as part of the predicate to describe an act of deception.
To, co powiedział, jest oszustwem.
What he said is a fraud.
Here, 'oszustwem' again in instrumental case refers to the act of fraud as an object in context.
Został oszukany, ale nie zgłaszał oszustwem policji.
He was deceived, but he did not report the fraud to the police.
The term 'oszustwem' is used in instrumental case to indicate the means by which the action is performed.