Abishag was a young servant woman who was given to attend to the then invalid King David for a brief time in his old age. After David died, one of David's sons, Adonijah, who had earlier tried to usurp the kingship for himself from the God-commanded rightful heir (his brother Solomon), asked to marry Abishag - a request that was recognized by Solomon as a part of another of Adonijah's deadly ploys to become king. In self defense, Solomon had his politically treacherous brother Adonijah killed.
"they sought for a fair damsel throughout all the coasts of Israel, and found Abishag"
Abishag was an intimate servant of the king, very nearly a wife or concubine.
"Now king David was old and stricken in years; and they covered him with clothes, but he gat no heat. Wherefore his servants said unto him, Let there be sought for my lord the king a young virgin: and let her stand before the king, and let her cherish him, and let her lie in thy bosom, that my lord the king may get heat."
"So they sought for a fair damsel throughout all the coasts of Israel, and found Abishag a Shunammite, and brought her to the king. And the damsel was very fair, and cherished the king, and ministered to him: but the king knew her not." (1 Kings 1:3-4 KJV)
Adonijah was the fourth-born son of David. After the death of his older brothers, Amnon and Absalom, Adonijah assumed himself to the throne. But The Lord had already chosen Solomon, Adonijah's younger brother, to succeed David. Nevertheless, Adonijah had himself proclaimed king while David was even yet living. Nathan and Bathsheba brought the situation to David's attention; David then declared Solomon king, even though David was himself yet alive.
"And Bathsheba went in unto the king into the chamber: and the king was very old; and Abishag the Shunammite ministered unto the king. And Bathsheba bowed, and did obeisance unto the king."
"And the king said, What wouldest thou?"
"And she said unto him, My lord, thou swarest by The Lord thy God unto thine handmaid, saying, Assuredly Solomon thy son shall reign after me, and he shall sit upon my throne. And now, behold, Adonijah reigneth; and now, my lord the king, thou knowest it not" (1 Kings 1:15-18 KJV)
After his first plot failed, Adonijah then asked Bathsheba to ask Solomon for Abishag in marriage.
"And Adonijah the son of Haggith came to Bathsheba the mother of Solomon. And she said, Comest thou peaceably?"
"And he said, 'Peaceably.' He said moreover, 'I have somewhat to say unto thee.'"
"And she said, 'Say on.'"
"And he said, Thou knowest that the kingdom was mine, and that all Israel set their faces on me, that I should reign: howbeit the kingdom is turned about, and is become my brother's: for it was his from The Lord. And now I ask one petition of thee, deny me not."
"And she said unto him, Say on."
"And he said, Speak, I pray thee, unto Solomon the king, for he will not say thee nay, that he give me Abishag the Shunammite to wife." (1 Kings 2:13-17 KJV)
Solomon's reaction, although often misinterpretted by many, was simply a matter of self defense. Solomon knew that his life would always be in danger with Adonijah lurking about. It was Solomon that The Lord declared was to be David's successor, and Solomon had a responsibility to fulfill The Lord's will. When Bathsheba delivered Adonijah's request to Solomon, Solomon did what was necessary to defend the throne.
"And she said, Let Abishag the Shunammite be given to Adonijah thy brother to wife."
"And king Solomon answered and said unto his mother, And why dost thou ask Abishag the Shunammite for Adonijah? Ask for him the kingdom also; for he is mine elder brother; even for him, and for Abiathar the priest, and for Joab the son of Zeruiah. Then king Solomon sware by The Lord, saying, God do so to me, and more also, if Adonijah have not spoken this word against his own life. Now therefore, as The Lord liveth, which hath established me, and set me on the throne of David my father, and who hath made me an house, as he promised, Adonijah shall be put to death this day."
"And king Solomon sent by the hand of Benaiah the son of Jehoiada; and he fell upon him that he died." (1 Kings 2:21-25 KJV)
Fact Finder: Adonijah was not the only son who tried to usurp the kingship from the rightful heir of that royal dynasty of Judah, of which Jesus Christ is the lawful descendant (see also Israelite Dynasties). What other son of David also tried it, by a military coup against his own father?