IPA: /kəʊˈvɜːtli/
KK: /ˈkoʊvɜrtli/
In a way that is not openly acknowledged or displayed; done in secret.
The spy covertly gathered information without anyone noticing.
Covertly → It is formed from "covert" (from Old French *covert*, meaning covered or concealed) and "-ly" (meaning in a manner of). The word "covertly" means in a manner that is concealed or hidden.
Think of something being 'covered' ('covert') and done 'in a manner' ('-ly') that is not easily seen — that's why covertly means doing something secretly.