From BiblePortal Wikipedia

Fausset's Bible Dictionary [1]

An Idumean, chief of Saul's herdsmen. At Nob ( 1 Samuel 21:7) "detained before the Lord" by some act of purification or vow, which as a proselyte he was performing, when Ahimelech gave David Goliath's sword and the shewbread. With officious eagerness and talebearing exaggeration (marked in the title of Psalm 52 by the tautology "came and told and said") he gave information which he knew well his master Saul would keenly listen to. Doeg told substantially the fact; it was Saul who put on it the "lying" construction of treason on the part of the priests (compare  Psalms 52:3-4 with  1 Samuel 22:13).

"The Edomite" in the title reminds us that herein Doeg represented Edom's and the world's undying enmity to Israel and the godly. He was but the accomplice and ready tool; Saul, the "mighty man" ( Psalms 52:1) who "trusted in the abundance of his riches" ( Psalms 52:7) as means of destroying David, was the real" boaster in mischief," for this was the very appeal that Saul made, and that induced Doeg to inform ( 1 Samuel 22:7): "Hear now, ye Benjamites, will the son of Jesse (as I can) give every one of you fields and vineyards?" (compare  1 Samuel 8:14.)

On Doeg's information, and by Doeg's own sacrilegious hand, at Saul's command, when the king's "footmen" declined in reverential awe to kill Jehovah's priests, eighty-five of them fell, and Saul "boasted" ( Psalms 52:1) of it as a sample of the fate of all who should help David. The undesigned coincidences here noted, between the psalm and independent history, confirm the authenticity of both. The cruel sycophancy of Doeg was so well known to David that he said unto Abiathar, the only survivor of the slaughter, "I knew it that day, when Doeg the Edomite was there, that he would surely tell Saul;" therefore with characteristic sensitiveness of conscience David adds, "I have occasioned the death of all the persons of thy father's house."

American Tract Society Bible Dictionary [2]

An Edomite, overseer of Saul's flocks. At Nob he witnessed the relief kindly furnished to David when fleeing from Saul, by Ahimelech the high priest, and carried a malicious and distorted report of it to his master. The king gladly seized the opportunity to wreak his passion on a helpless victim; and when the Jews around him refused to slay the priests of God, infamously used the willing services of this alien and heathen. Doeg not only slew Ahimelech and eighty-four other priests, but put the town in which they dwelt to the sword,  1 Samuel 21:15 . David forebodes his wretched fate,  Psalm 52:1-9   120:1-7   140:1-13 .

People's Dictionary of the Bible [3]

Doeg ( Dô'Eg ), Fearful. An Edomite, the chief of Saul's herdsmen, "detained before the Lord," probably by a vow, or because it was the sabbath, when David fled to Nob.  1 Samuel 21:7. Doeg afterwards falsely accused Abimelech, the high priest, to Saul; and, when none of the king's guard would execute the ferocious sentence to slay the priests of the Lord, he fell upon them and killed 80 persons, sacking also their city.  1 Samuel 22:9-19;  Psalms 52:1-9, title.

Hastings' Dictionary of the Bible [4]

DOEG . An Edomite, and chief of the herdmen [or better, ‘runners,’ reading hâ-râtsîm for hâ-rô‘îm ] of king Saul. When David fled to Nob to Ahimelech (or Ahijah) the priest, Doeg was there ‘detained before the Lord.’ Upon his report Saul ordered Ahimelech and his companions to be slain. The order was carried out by Doeg, when the rest of the king’s guard shrank from obeying it (  1 Samuel 21:7;   1 Samuel 22:9-19 ). Doeg is mentioned in the title of   Psalms 52:1-9 .

Smith's Bible Dictionary [5]

Do'eg. (Fearful). An Idumean, chief of Saul's herdmen. (B.C. 1062). He was at Nob when Ahimelech gave David the sword of Goliath, and not only gave information to Saul, but when others declined the office, himself executed the king's order to destroy the priests of Nob, with their families, to the number of 85 persons, together with all their property.  1 Samuel 21:7;  1 Samuel 22:9;  1 Samuel 22:18;  1 Samuel 22:22; Psalms 52.

Morrish Bible Dictionary [6]

Chief of Saul's herdsmen, an Edomite, who informed Saul of David's being aided by Ahimelech, and who afterwards slew the latter and his house — 85 priests.  1 Samuel 21:7;  1 Samuel 22:9-22;  Psalm 52 title . We have no information as to how such a man could have been 'detained before the Lord.' He may have been a proselyte and had some vow upon him.

Holman Bible Dictionary [7]

 1 Samuel 21:7 Psalm 52:1Saul

Easton's Bible Dictionary [8]

 1 Samuel 21:7 Psalm 52

International Standard Bible Encyclopedia [9]

dō´eg ( דּואג , דּאג , dō' - ēgh "anxious," "cared for"): "The Edomite," a servant of Saul, who watched David's intercourse with the priest Ahimelech, then denounced the priest to the king, and later executed his command to slay the priests at Nob. The position he held is described as that of "the mightiest" of Samuel's herdsmen ( 1 Samuel 21:7 margin). Septuagint reads: "tending the mules." Rabbinical legends speak of him as the greatest scholar of his time. The traditional title of   Psalm 52:1-9 associates the composition of that Psalms with the events that led to the slaying of the priests (  1 Samuel 21:7;  1 Samuel 22:9 ,  1 Samuel 22:18 ,  1 Samuel 22:22 ).

Kitto's Popular Cyclopedia of Biblial Literature [10]

Do´eg, an Edomite, and chief overseer of king Saul's flocks, which is an important trust in Oriental courts. At Nob he was witness of the assistance which the high-priest Ahimelech seemed to afford to the fugitive David, by furishing him with the sword of Goliath, and by supplying him with bread even from the sacred table . Of this he failed not to inform the king, who, regardless of the explanation offered by Ahimelech, and finding that the chiefs censured him, and hesitated to lay their hands upon a person so sacred, commanded Doeg to slay him and his priests—a task which was executed with equal readiness and cruelty by the Edomite (, sqq.).

The Nuttall Encyclopedia [11]

A herdsman of Saul (1Sam. xxi. 7); a name applied by Dryden to Elkanah Settle in "Absalom and Achitophel."