The Greek verb mainomai means to be mad, to rave, or to be out of one's mind — used both of actual insanity and of the accusation of madness leveled at those whose words seem impossible or whose devotion seems excessive.
Mainomai appears in several defining moments of the New Testament. Festus shouts at Paul, 'You are out of your mind (mainomai)! Your great learning is driving you insane!' (Acts 26:24) — and Paul's calm, reasoned response is one of the great defenses of the faith. The disciples are accused of being drunk (mainomai implied) at Pentecost (Acts 2:13). Unbelievers told that a dead person was alive called the report madness: when Rhoda announces Peter is at the door, the church replies 'You are out of your mind' (mainomai, Acts 12:15). Paul even anticipates that tongues-speaking without interpretation will make outsiders say 'you are out of your mind' (1 Corinthians 14:23). The gospel has always appeared like madness to those who have not yet received it.