IPA: /ˈtʃɪəfəl.nəs/
KK: /ˈtʃɪrflnəs/
The state of being happy and positive.
Her cheerfulness brightened everyone's day.
Cheerfulness is formed from "cheer" (from Old French *chiere*, meaning face or countenance, which evolved to mean mood or spirit) and "-fulness" (meaning the state of being full of). The word "cheerfulness" means the state of being full of cheer or good spirits.
Think of 'cheer' as representing a happy mood and '-fulness' as being full of that mood. This helps you remember that cheerfulness means being full of cheer.