This is the 12505th most frequent Polish word.
najstarsza
"Najstarsza" means "the oldest" in English.
Here, 'najstarsza' modifies 'książka', meaning 'the oldest book', indicating possession of the quality of being old to the highest degree among comparable items.
Najstarsza książka w bibliotece jest bardzo wartościowa.
The oldest book in the library is very valuable.
In this sentence, 'najstarsza' modifies 'siostra', meaning 'the oldest sister', referring to the sibling who is the most advanced in age relative to the others.
Moja najstarsza siostra właśnie skończyła studia.
My oldest sister has just graduated from university.
Here, 'najstarsza' refers to a mountain that is considered the oldest one in Poland, denoting a comparison based on age among mountains.