This is the 12286th most frequent Polish word.
minionym
"Minionym" means "bygone" or "past."
Here, 'minionym' is used as an adjective in the instrumental case to describe 'roku', indicating the previous year.
W minionym roku wiele się wydarzyło.
In the past year, many things happened.
In this context, 'minionym' modifies 'wieczorem' in the instrumental case to describe a specific evening that has already passed.
Minionym wieczorem rozmawialiśmy o naszych planach.
Last evening, we talked about our plans.
Here, 'minionych' (the plural form of 'minionym' in the genitive case) is used to modify 'miesięcy', indicating a group of prior months.