This is the 10685th most frequent German word.
riesen
"Riesen" means "giants" in English.
The word 'riesen' is used as an adjective emphasizing the size of the apple, denoting 'giant'.
Das ist ein riesen Apfel.
That is a giant apple.
The word 'Riesen' acts as a noun referring to a mythical or extraordinary being of large stature, 'giant'.
Ich habe einen Riesen getroffen.
I encountered a giant.
The word 'riesen' is used as an adjective in an idiomatic structure, intensifying the challenge implied.