ARM. Used as a figure for personal, active power. Thus the Lord lays “bare his holy arm” (Isa.52.10), rather as we might say of someone about to undertake some task, “he rolled up his sleeves.” The Lord’s arm (Isa.53.1) is figurative of his personal intervention. In particular the figure of the “arm” looks back to what the Lord did at the Exodus (Exod.6.6; Deut.4.34; Deut.5.15; Isa.51.9-Isa.51.11).

International Standard Bible Encyclopedia (1915)

The usual form is zeroa` from the root zara`, "to spread." The arm may be "stretched out." ’Ezroa` is this form with prosthetic ’aleph (Job 31:22; Jer 32:21), and dera` is the Aramaic form. Chotsen is really "bosom," thus the Revised Version (British and American) (Isa 49:22); and katheph is "shoulder," thus the Revised Version (British and American) (Job 31:22). Compare cheir, also, in Ac 11:21.

Figurative: The arm denotes influence, power, means of support or conquest. The arms of Moab (Jer 48:25) and of Pharaoh (Eze 30:21 ff) are broken. The arm of Eli and the arm of his father’s house are to be cut off (1Sa 2:31). Because the arm wielded the sword it signified "oppression" (Job 35:9). The arms are the means of support, therefore to refuse to aid the fatherless is to break their arms (Job 22:9).