To cleanse is to clean. It is usually referred to for cleaning energy instead of physical cleaning.
The word cleanse is inherited from Germanic. Germanic predates Old English and is a parent language. One of the Old English spellings is clǽnsian. Many words in Old English had endings such as -n. We also see an old letter that is now obsolete, the ash, ǽ. Over time, through Middle English, the -n fell off and we see that the spelling becomes more standardized.
Sources:
“Cleanse, V.” Oxford English Dictionary, Oxford UP, July 2023, https://doi.org/10.1093/OED/2773068541.