IPA: /ˈhɪdənli/
KK: /ˈhɪdənli/
In a way that is not visible or easily noticed.
The treasure was hiddenly placed under the old oak tree.
Hiddenly → It is formed from "hidden" (from Old English "hydan", meaning to conceal) and "-ly" (a suffix meaning in a manner of). The word "hiddenly" means in a manner that is concealed or not visible.
Think of something that is 'concealed' ('hidden') and done 'in a manner' ('-ly'). This helps you remember that 'hiddenly' refers to doing something in a concealed way.