The Greek diabolos combines dia (through/across) and ballo (to throw) to mean 'one who throws across' or 'one who accuses'. As a noun with the article (ho diabolos), it is the primary Greek title for Satan, the Devil. As an adjective, it means slanderous.
Diabolos captures the devil's essential nature: he is the Accuser (cf. Hebrew satan, 'adversary'). The devil's primary tactic is slander — bringing charges against God's people before the heavenly court (Job 1–2; Zechariah 3:1; Revelation 12:10). Jesus identifies him as 'the father of lies' (John 8:44) and 'a murderer from the beginning' — the liar who convinced Eve that God's command was a lie, triggering death. Against this accuser stands the Advocate: Jesus Christ 'the righteous' (1 John 2:1), whose atoning blood silences every charge (Romans 8:33–34). Revelation 12:10–11 celebrates the defeat of 'the accuser of our brothers' by 'the blood of the Lamb and the word of their testimony.' The Christian life is lived in the confidence that every accusation against us has been answered at the cross.