IPA: /ˈsiːkrɪtli/
KK: /ˈsikrɪtli/
In a way that is not known or seen by others; done without others being aware.
She secretly planned a surprise party for her friend.
Secretly → It is formed from "secret" (from Latin "secretus", meaning set apart or hidden) and "-ly" (meaning in a manner of). The word "secretly" means in a manner that is hidden or not openly revealed.
Think of something that is 'set apart' ('secretus') and done 'in a manner' ('-ly') that is hidden — that's why secretly means to do something in a hidden way.