This is the 1138th most frequent Dutch word.
rot
"Rot" in Dutch means "decay" or "something rotten."
The word "rot" is used here as an adjective meaning "rotten", describing the state of the apple.
De appel is helemaal rot geworden.
The apple has become completely rotten.
In this sentence, "rot" is part of the compound word "rotstreek", where it adds an emotional connotation meaning unpleasant or bad.
Wat een rotstreek om te verdwijnen zonder iets te zeggen!
What a dirty trick to disappear without saying anything!
Here, "rot" is used informally as an adjective to describe a negative emotional state or feeling unwell.