This is the 16102nd most frequent Polish word.
uczciwej
"Uczciwej" means "honest" or "fair" in English (feminine singular genitive or dative form).
The word 'uczciwej' here is an adjective in genitive case (feminine) modifying the noun 'przyjaźni' (friendship).
To był akt uczciwej przyjaźni.
That was an act of honest friendship.
In this context, 'uczciwej' is an adjective in genitive case (feminine), describing the noun 'oferty' (offer).
Nie mogę zaakceptować uczciwej oferty.
I cannot accept an honest offer.
'Uczciwej' modifies the noun 'decyzji' (decision), clarifying its quality as honest, and is in genitive case to match the subject role.