This is the 317th most frequent Lithuanian word.
mėnesių
"Months."
Here, 'mėnesių' is used as the genitive plural form of 'mėnuo', representing an unspecified number of months.
Aš gyvenau miesto centre trejus mėnesius.
I lived in the city center for three months.
In this case, 'mėnesių' is again the genitive plural, describing the time frame in which the changes occurred.
Per pastaruosius kelis mėnesius daug dalykų pasikeitė.
Many things have changed during the last few months.
Here, 'mėnesių' is the genitive plural used to indicate the duration of preparation expressed by 'keletą', meaning 'a few'.