IPA: /səˈdɪstɪkli/
KK: /sæˈdɪstɪkli/
In a way that shows enjoyment in causing pain or suffering to others.
He laughed sadistically as he watched the horror movie.
Sadistically → It is formed from "sadist" (from French *sadisme*, meaning the enjoyment of inflicting pain) and "-ically" (meaning in a manner relating to). The word "sadistically" means in a manner that relates to deriving pleasure from causing pain or suffering to others.
Think of someone who enjoys causing pain ('sadist') and remember that 'sadistically' describes doing something in that painful manner.