This is the 763rd most frequent Polish word.
pierwsza
"Pierwsza" means "first" in English (feminine form).
'Pierwsza' is used here as an adjective describing a feminine noun 'myśl', meaning 'first'.
Pierwsza myśl była najlepsza.
The first thought was the best.
'Pierwsza' functions as a predicate adjective here, referring to a feminine subject.
Była pierwsza na liście.
She was the first on the list.
'Pierwsza' describes the noun 'godzina', indicating its position in a sequence.