IPA: /ˈrɒtənli/
KK: /ˈrɑtənli/
In a way that is decayed or spoiled, often used to describe something that is unpleasant or morally bad.
The fruit was left out too long and now smells rottenly.
Rottenly → It is formed from "rotten" (from Old English *rotian*, meaning to decay or decompose) and "-ly" (meaning in a manner of). The word "rottenly" means in a manner that is decayed or decomposed.
Think of something that is 'decayed' ('rotten') and how it is done 'in a manner' ('-ly'). This helps you remember that 'rottenly' describes something done in a decayed way.