IPA: //ˈfʊlɪst//
KK: /fʊlɪst/
The highest degree of being filled or occupied; completely filled with something.
She lived her life to the fullest, enjoying every moment.
Fullest → It is formed from "full" (from Old English *full*, meaning filled or complete) and the suffix "-est" (meaning the most). The word "fullest" means the most filled or the maximum extent of fullness.
Think of something being 'filled' ('full') to the 'most' ('-est') — that's why fullest means the most filled.
No commonly confused words.