IPA: /ˈɡreɪtli/
KK: /ˈɡreɪtli/
To a large extent or degree; very much.
She was greatly impressed by the performance.
Greatly → It is formed from "great" (from Old English *grēat*, meaning large or big) and "-ly" (meaning in a manner of). The word "greatly" means in a manner that is large or significant.
Think of something being 'large' or 'big' ('great') and how it is done 'in a manner' ('-ly'). This helps you remember that 'greatly' means to do something in a significant way.