This is the 11006th most frequent Polish word.
fiasku
"Fiasku" means "failure" or "fiasco" in English.
Here, 'fiaskiem' is used as a noun in instrumental case to describe the unsuccessful end of plans.
Wszystkie nasze plany zakończyły się fiaskiem.
All our plans ended in failure.
In this sentence, 'fiasku' is used as a noun in genitive case to indicate the concept of failure.
Nie chcę przeżywać kolejnego fiasku w tej sprawie.
I don't want to experience another failure in this matter.
'Fiaskiem' here is used as a noun in instrumental case describing the project as a failure.