Fausset's Bible Dictionary
(See Psalms 41:9; Psalms 55:12-14; Psalms 55:20-21. His name means brother of foolishness, but his oracular wisdom was proverbial. David's prayer "turned his counsel" indeed into what his name indicated, "foolishness" (2 Samuel 15:31; Job 5:12-13; 1 Corinthians 1:20). Ahithophel was the mainspring of the rebellion. Absalom calculated on his adhesion from the first (2 Samuel 15:12); the history does not directly say why, but incidentally it comes out: he was father of Eliam (or by transposition Ammiel, 1 Chronicles 3:5), the father of Bathsheba (2 Samuel 11:3; 2 Samuel 23:34; 2 Samuel 23:39)..) Of Giloh, in the hill country of Judah. David's counselor, to whose treachery he touchingly alludes
Uriah the Hittite and Eliam, being both of the king's guard (consisting of 37 officers), were intimate, and Uriah married the daughter of his brother officer. How natural Ahithophel's sense of wrong toward David, the murderer of his grandson by marriage and the corrupter of his granddaughter! The evident undesignedness of this coincidence confirms the veracity of the history. The people's loyalty too was naturally shaken toward one whose moral character they had ceased to respect. Ahithophel's proposal himself to pursue David that night with 12,000 men, and smite the king only, indicates the same personal hostility to David, deep sagacity and boldness. He failed from no want of shrewdness on his part, but from the folly of Absalom. His awful end shows that worldly wisdom apart from faith in God turns into suicidal madness (Isaiah 29:14). He was the type of Judas in his treachery and in his end. (See .)
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Fausset, Andrew R. Entry for 'Ahithophel'. Fausset's Bible Dictionary. https://www.studylight.org/dictionaries/eng/fbd/a/ahithophel.html. 1949.