This is the 4626th most frequent Polish word.
minionej
"Minionej" means "past" or "previous" in English.
Here, 'minionej' is used to describe the 'decade' as the one that has passed, functioning as an adjective in genitive case.
W minionej dekadzie wydarzyło się wiele znaczących zmian.
In the past decade, many significant changes occurred.
In this sentence, 'minionej' qualifies the 'party' as one that already took place, showing a completed event.
Podczas minionej imprezy spotkałem wielu starych znajomych.
During the previous party, I met many old friends.
Here, 'minionej' is used to refer to 'school' in the sense that it is one that has been left behind or is no longer attended.