IPA: /ˈpeɪn.ləs.li/
KK: /ˈpeɪnləsli/
In a way that does not cause any discomfort or suffering.
The procedure was completed painlessly, leaving the patient feeling relieved.
Painlessly → It is formed from "pain" (from Old French *peine*, meaning suffering or discomfort) and "-less" (meaning without) and "-ly" (meaning in a manner). The word "painlessly" means in a manner that is without pain or discomfort.
Think of being 'without' ('-less') 'suffering' ('pain') — that's why doing something 'painlessly' means doing it without any discomfort.