The word "διάβολος" in ancient Greek was an adjective.
The three genders were διάβολος (masc), διάβολος (fem.), διάβολον (neut.) = slanderous, defametory.
It was also used as a noun "ο διάβολος" = the slanderer
Nowadays, in modern Greek, the word is used only as a noun and it means "the Devil"