IPA: /ˈniːdləsli/
KK: /nidlisli/
In a way that is not necessary or required; excessively or unnecessarily.
He spent needlessly on things he didn't really want.
Needlessly → It is formed from "need" (from Old English *nēod*, meaning necessity or requirement) and "-less" (meaning without). The word "needlessly" means without necessity or requirement, indicating something done that is not required.
Think of 'need' as something necessary and '-less' meaning without — so 'needlessly' means doing something without any necessity.