This is the 322nd most frequent Polish word.
energii
The Polish word "energii" means "energy" (genitive or dative form).
In this context, 'energii' is in the genitive case, used to denote the lack of something—here, energy.
Odczuwam brak energii po długim dniu pracy.
I feel a lack of energy after a long day of work.
Here, 'energii' is in the genitive case, used to express possession or a need.
Potrzebuję więcej energii, żeby ukończyć to zadanie.
I need more energy to complete this task.
'Energii' in this sentence is also in the genitive case, referring to the category or type of 'sources'.