From what I can tell, "Esuna" isn't a Japanese word at all.

If I had to hypothesize on what it means, the obvious part is that the suffix "na" can mean to undo something; for example, in FFI, the spell Blina is short for "Blind-na," basically meaning "Antiblind." Similarly, Poisona means "Antipoison," and Stona is "Antistone."

The only real question, then, is where the "esu" comes from. Now, this is pure conjecture, but it could indicate the pronounciation of the Roman letter "S," making it "S-na." "S," then, could stand for "status," which would mean "Esuna" means "Antistatus."