Concise Bible Dictionary:

Beside the common signification of this term for a male relative, it is used typically in reference to the Lord Jesus in His relationship with Israel. As their kinsman He has the right of redemption, and will undertake their cause in a future day, as Boaz did the cause of Naomi and Ruth (Ruth 2-4).

From Manners and Customs of the Bible:

1. The expression “spread thy skirt” imports protection, and here signifies protection of a conjugal character. When marriages are solemnized among the Jews the man throws the skirt of his talith or robe over his wife and covers her head with it.
2. Goel, “kinsman,” is, literally, “one who redeems.” When a Hebrew was obliged to sell his inheritance on account of poverty, it was the duty of the nearest relative to redeem it for him (Lev. 25:2525If thy brother be waxen poor, and hath sold away some of his possession, and if any of his kin come to redeem it, then shall he redeem that which his brother sold. (Leviticus 25:25)). Hence the word goel came to signify kinsman. The goel also became the recipient of property which had been unjustly kept from a deceased kinsman (Num. 5:6-86Speak unto the children of Israel, When a man or woman shall commit any sin that men commit, to do a trespass against the Lord, and that person be guilty; 7Then they shall confess their sin which they have done: and he shall recompense his trespass with the principal thereof, and add unto it the fifth part thereof, and give it unto him against whom he hath trespassed. 8But if the man have no kinsman to recompense the trespass unto, let the trespass be recompensed unto the Lord, even to the priest; beside the ram of the atonement, whereby an atonement shall be made for him. (Numbers 5:6‑8)). It was likewise his duty to avenge the blood of his next of kin by seeking the life of the murderer (Genesis 9:5-65And surely your blood of your lives will I require; at the hand of every beast will I require it, and at the hand of man; at the hand of every man's brother will I require the life of man. 6Whoso sheddeth man's blood, by man shall his blood be shed: for in the image of God made he man. (Genesis 9:5‑6); Num. 35:1919The revenger of blood himself shall slay the murderer: when he meeteth him, he shall slay him. (Numbers 35:19); 2 Sam. 14:77And, behold, the whole family is risen against thine handmaid, and they said, Deliver him that smote his brother, that we may kill him, for the life of his brother whom he slew; and we will destroy the heir also: and so they shall quench my coal which is left, and shall not leave to my husband neither name nor remainder upon the earth. (2 Samuel 14:7)).
Some have supposed from the association of the goel with marriage, as in this history of Ruth, that it was his duty to marry the widow of a deceased kinsman: but according to Deuteronomy 25:55If brethren dwell together, and one of them die, and have no child, the wife of the dead shall not marry without unto a stranger: her husband's brother shall go in unto her, and take her to him to wife, and perform the duty of an husband's brother unto her. (Deuteronomy 25:5), this duty was only obligatory on a brother-in-law, which relation to Ruth was certainly not sustained by Boaz. Nor is there any evidence that it was sustained by the unnamed kinsman spoken of by Boaz in verse 12. Had this nearer goel been a brother-in-law Boaz would not have begun by asking him to redeem the property (Ruth 4:44And I thought to advertise thee, saying, Buy it before the inhabitants, and before the elders of my people. If thou wilt redeem it, redeem it: but if thou wilt not redeem it, then tell me, that I may know: for there is none to redeem it beside thee; and I am after thee. And he said, I will redeem it. (Ruth 4:4)) but would instantly have demanded that he should marry the widow, on refusing to do which he was liable to judicial disgrace (Deut. 25:7-107And if the man like not to take his brother's wife, then let his brother's wife go up to the gate unto the elders, and say, My husband's brother refuseth to raise up unto his brother a name in Israel, he will not perform the duty of my husband's brother. 8Then the elders of his city shall call him, and speak unto him: and if he stand to it, and say, I like not to take her; 9Then shall his brother's wife come unto him in the presence of the elders, and loose his shoe from off his foot, and spit in his face, and shall answer and say, So shall it be done unto that man that will not build up his brother's house. 10And his name shall be called in Israel, The house of him that hath his shoe loosed. (Deuteronomy 25:7‑10)). But in the case of the god it was not until he redeemed the property of his relative, dying without a son, that he was under obligation to marry the widow. As Winer says, “The latter was to him the consequence of the former and not the reverse, as in the case of the levir, [brother-in-law.] Should he refuse to take possession of the property he was under no obligation to marry the widow. In so refusing he incurred no judicial disgrace, because he did not fail to discharge a duty, but only relinquished a right. The law had expressly imposed the duty of marriage on the levir only, and beyond him the obligation did not extend” (Real Wörterbauch, s. v. Ruth).
Boaz had no right to redeem the property until the nearer kinsman refused, and neither he nor the other kinsman was under any obligation to do it; but having once assumed the redemption, the one thus exercising his right was by that act under obligation to marry the widow.

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