History and Etymology
diabolical from
diabolic +
-ical;
diabolic going back to Middle English
deabolik, borrowed from Middle French & Late Latin; Middle French
dyabolique, borrowed from Late Latin
diabolicus, borrowed from Late Greek
diabolikós, going back to Greek, "slanderous," from
diábolos "accuser, backbiter, slanderer" +
-ikos -ic entry 1 — more at
devil entry 1