IPA: /dɪˈfɛnsəslɪ/
KK: /dɪˈfɛnsəslɪ/
In a way that shows a lack of protection or defense against harm or attack.
The child stood defenselessly in front of the bully, unable to defend himself.
Defenselessly → It is formed from "defenseless" (from "defense" which comes from Latin "defensio" meaning protection, and the suffix "-less" meaning without) and "-ly" (meaning in a manner of). The word "defenselessly" means in a manner that is without protection or defense.
Think of being 'without' ('-less') 'protection' ('defense') — that's why defenselessly means acting in a way that lacks protection.