IPA: /ˈhɑːrmləsli/
KK: /ˈhɑrm.ləs.li/
In a way that does not cause harm or injury.
The chemicals were used harmlessly in the experiment.
Harmlessly → It is formed from "harm" (from Old English "hearm", meaning injury or damage) and "-less" (meaning without) and "-ly" (meaning in a manner). The word "harmlessly" means in a manner that is without harm or injury.
Think of being 'without harm' ('-less') and doing something 'in a manner' ('-ly') that causes no injury.