Hastings' Dictionary of the Bible (1898)
MAD is the translation of various Hebrew and Greek words, sometimes denoting actual insanity, 1 Sam 21:13-15; 1 Cor 14:23, but generally signifying an uncontrollable excitement only, caused either by fierce wrath, Deut 28:28, Num 32:34; 2 Kgs 9:11; Luke 6:11; Acts 26:11, or the frenzy of idolatrous worship, 1 Kgs 18:26, 1 Kgs 18:28, or real inspiration, 1 Sam 19:21-24; comp. 2 Kgs 9:11; Jer 29:26; Acts 2:13. Only once in Scripture is madness connected with demoniacal possession. John 10:20.
Among the Orientals, as among all semi-civilized people, madness was generally looked upon partly with pity, because God's hand was laid heavily on the madman, partly with reverence, because the mad mind, being shut up against all ordinary impressions, was considered open to supernatural or spiritualistic influences. Thus it became possible for David to effect his escape from the court of Achish by feigning madness. 1 Sam 21:13-15.