IPA: /ˈwʌndərfəl.nəs/
KK: /ˈwʌndərfəl.nəs/
The quality or state of being extremely good, impressive, or admirable.
The wonderfulness of the sunset took my breath away.
Wonderfulness → It is formed from "wonder" (from Old English *wundor*, meaning a marvel or something extraordinary) and "-fulness" (meaning the state of being full of). The word "wonderfulness" means the state of being full of wonder or marvel.
Think of something that is a 'marvel' ('wonder') and is 'full of' ('-fulness') that quality — that's why wonderful means full of wonder.