This is the 15210th most frequent Polish word.
sentymentalnego
"Sentymentalnego" means "sentimental" in English.
The word 'sentymentalnego' is used here as an adjective in the genitive case to describe the noun 'dzieciństwa' (childhood).
Ten film ma nostalgiczny charakter i przypomina mi czasy sentymentalnego dzieciństwa.
This film has a nostalgic character and reminds me of a sentimental childhood.
'Sentymentalnego' in this context describes 'znaczenia' (significance) and fits the genitive case for adjectives.
Twój prezent dla babci był naprawdę gestem sentymentalnego znaczenia.
Your gift for grandma was truly a gesture of sentimental significance.
Here, 'sentymentalnego' modifies 'tonu' (tone), applying the genitive case for a descriptive term.