This is the 442nd most frequent German word.
jüngsten
"Jüngsten" means "youngest" or "most recent" in English, depending on context.
'Jüngsten' in this case is the genitive singular of the adjective 'jüngst', meaning 'youngest'. It modifies 'Kindes' (child's).
Das jüngsten Kindes Lächeln erfreute alle.
The youngest child's smile delighted everyone.
'Jüngsten' here modifies 'Jahren' (years), working as an adjective in dative plural to describe the proximity in time.
In den jüngsten Jahren hat sich viel verändert.
In the most recent years, a lot has changed.
'Jüngsten' here is an adjective meaning 'recent', modifying the plural noun 'Ereignisse' (events). It conveys the concept of being close in time.