In Bunyan's Holy War: a member of the host of Diabolus (the Devil) in his assault upon Mansoul. More generally: a disciple or worshipper of the Devil; an apostate.
Originally: of, belonging to, or relating to Diabolus or his followers. More generally: that is a disciple or worshipper of the Devil; of, relating to, or consisting of disciples of the Devil.
Late 17th century; earliest use found in John Bunyan (d. 1688), author. From post-classical Latin diabolus devil + -onian.