“Esssse” is an "obsolete" form of the word “ashes.” It is in the OED (2nd edition).
This word is noticeably a palindrome and can be used to form a 6x6 palindromic square (according to the website where I found it, this palindromic square was created by someone named Daniel Austin), which looks like a 6x6 square (of course) with the following words stacked seriatim:
Personally, I don’t think that square should be valid, since “sevres” should actually be spelled “Sèvres.” Oh well.
Primarily, this word is just cool because it has four of the same consonant in a row. I believe it is the only such word in the English language.
I have not yet been able to find a reliable source as to the pronunciation of this incredibly cool word. The OED does not provide a pronunciation for it. When I searched for it, the few websites that did give some sort of pronunciation claimed that it is pronounced “eshih.” That’s not particularly helpful, since I can think of at least four ways to pronounce “eshih.” If anyone can find a better pronunciation guide for this word, please let me know.
says: Probably this word had a long 'sh' sound, as could happen in Middle English, so the 'ss' was written twice. The final 'e' is a schwa, like the last sound of sofa: that makes it like 'esha' but with a prolonged 'sh'.