This is the 1044th most frequent Polish word.
armii
The word "armii" in Polish means "of the army" (genitive singular) or "to/for the army" (dative singular).
Here, 'armii' is used in the genitive case to specify possession, meaning 'of the army'.
Żołnierze armii wracają do koszar po ćwiczeniach.
Soldiers of the army are returning to the barracks after exercises.
In this sentence, 'armii' is used in the dative case to indicate the indirect object, meaning 'to the army'.
Dołącz do armii i służ swojej ojczyźnie.
Join the army and serve your homeland.
Here, 'armii' is used in the locative case to indicate the location or subject matter, meaning 'about the army'.