Concise Bible Dictionary:

The member of the body which is capable of lifting burdens and defending the person: it is used symbolically for the power and strength of God on behalf of His saints (Ex. 15:1616Fear and dread shall fall upon them; by the greatness of thine arm they shall be as still as a stone; till thy people pass over, O Lord, till the people pass over, which thou hast purchased. (Exodus 15:16); Psa. 77:1515Thou hast with thine arm redeemed thy people, the sons of Jacob and Joseph. Selah. (Psalm 77:15); Isa. 51:99Awake, awake, put on strength, O arm of the Lord; awake, as in the ancient days, in the generations of old. Art thou not it that hath cut Rahab, and wounded the dragon? (Isaiah 51:9); Isa. 53:11Who hath believed our report? and to whom is the arm of the Lord revealed? (Isaiah 53:1)). The arm of Jehovah is often spoken of in the Old Testament It redeemed (Ex. 6:66Wherefore say unto the children of Israel, I am the Lord, and I will bring you out from under the burdens of the Egyptians, and I will rid you out of their bondage, and I will redeem you with a stretched out arm, and with great judgments: (Exodus 6:6)); gathers His own (Isa. 40:1111He shall feed his flock like a shepherd: he shall gather the lambs with his arm, and carry them in his bosom, and shall gently lead those that are with young. (Isaiah 40:11)); and rules for Him (Isa. 40:1010Behold, the Lord God will come with strong hand, and his arm shall rule for him: behold, his reward is with him, and his work before him. (Isaiah 40:10)), as in the kingdom. It is a holy arm (Isa. 52:1010The Lord hath made bare his holy arm in the eyes of all the nations; and all the ends of the earth shall see the salvation of our God. (Isaiah 52:10); Psa. 98:11<<A Psalm.>> O sing unto the Lord a new song; for he hath done marvellous things: his right hand, and his holy arm, hath gotten him the victory. (Psalm 98:1)); and it is a glorious arm (Isa. 63:1212That led them by the right hand of Moses with his glorious arm, dividing the water before them, to make himself an everlasting name? (Isaiah 63:12)). The arm of the Lord is revealed to souls where there is repentance and faith in the report which God sends (Isa. 53:11Who hath believed our report? and to whom is the arm of the Lord revealed? (Isaiah 53:1); Rom. 10:1616But they have not all obeyed the gospel. For Esaias saith, Lord, who hath believed our report? (Romans 10:16)). It is to be trusted in even by the isles of the Gentiles, that is, by sinners everywhere in creation (Isa. 51:55My righteousness is near; my salvation is gone forth, and mine arms shall judge the people; the isles shall wait upon me, and on mine arm shall they trust. (Isaiah 51:5)).

From Manners and Customs of the Bible:

As the arm is an emblem of power, so shortness of arm signifies diminished power, and length of arm an increase. Thus it is said that Artaxerxes Longimanus, that is, the Long-handed, was so called, not because of any peculiarity of body, but because of the vast extent of his power. There is an ancient Egyptian sculpture in which the same bold figure is employed as the one used by the prophet in the text. It represents Thannyras, the son of Inarus, whom Artaxerxes had made Satrap of Egypt, worshiping the sun as his god. In this he disregarded the religion of his own people, and adopted that of their conquerors. The sun is represented as sending his rays down on the earth, and at the end of every ray is a hand.

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