IPA: /ˈspɒt.ləs.li/
KK: /ˈspɑt.ləs.li/
In a way that is completely clean and free from dirt or stains.
The kitchen was spotlessly clean after the cleaning crew finished their work.
Spotlessly → It is formed from "spotless" (from Old English "spotleas", meaning without spots or blemishes) and "-ly" (meaning in a manner of). The word "spotlessly" means in a manner that is free from spots or blemishes.
Think of being 'without spots' ('spotless') and how the '-ly' indicates the manner — so to be spotlessly clean means to be clean in a way that has no spots.