This is the 2654th most frequent Polish word.
balu
The Polish word "balu" means "of the ball" (as in a dance or formal event).
Here, 'balu' is used in the genitive case of 'bal' to specify the memories originating from or related to the ball.
Nie zapomnę tych wspomnień z wielkiego balu.
I won't forget these memories from the grand ball.
The word 'balu' here is a dative case of 'bal' (ball), suggesting the location or direction towards which an action is oriented.
Idę dzisiaj wieczorem na balu.
I am going to a ball this evening.
In this sentence, 'balu' is the genitive case of 'bal', used to indicate possession or association of the described situation happening at the ball.