IPA: //ˈsiːs.ləs.li//
KK: /ˈsiːs.ləs.li/
In a way that does not stop or pause; continuously.
The rain fell ceaselessly throughout the night.
Ceaselessly → It is formed from "cease" (from Old French *ceser*, meaning to stop) and the suffix "-less" (meaning without) combined with the adverbial suffix "-ly" (meaning in a manner). The word "ceaselessly" means in a manner that is without stopping or continuously.
Think of 'cease' meaning to stop, and remember that 'ceaselessly' means doing something without stopping.