What does arm mean?
The definition of arm
Arm, n.
Etym: [AS. arm, earm; akin to OHG. aram, G., D., Dan., & Sw. arm, Icel. armr, Goth. arms, L. armus arm, shoulder, and prob. to Gr. rame. Art, Article.]

1 The limb of the human body which extends from the shoulder to the hand; also, the corresponding limb of a monkey.
2 Anything resembling an arm; as,

(a) The fore limb of an animal, as of a bear.
(b) A limb, or locomotive or prehensile organ, of an invertebrate animal.
(c) A branch of a tree.
(d) A slender part of an instrument or machine, projecting from a trunk, axis, or fulcrum; as, the arm of a steelyard.
(e) (Naut) The end of a yard; also, the part of an anchor which ends in the fluke.
(f) An inlet of water from the sea.
(g) A support for the elbow, at the side of a chair, the end of a sofa, etc.

3 Fig.: Power; might; strength; support; as, the secular arm; the arm of the law. To whom is the arm of the Lord revealed Isa. lii. 1. Arm’s end, the end of the arm; a good distance off. Dryden.

— Arm’s length, the length of the arm.
— Arm’s reach, reach of the arm; the distance the arm can reach.
— To go (or walk) arm in arm, to go with the arm or hand of one linked in the arm of another. “When arm in armwe went along.” Tennyson.
— To keep at arm’s length, to keep at a distance (literally or figuratively); not to allow to come into close contact or familiar intercourse.
— To work at arm’s length, to work disadvantageously.

Arm, n.
Etym: [See Arms.] (Mil.)
(a) A branch of the military service; as, the cavalry arm was made efficient.
(b) A weapon of offense or defense; an instrument of warfare; — commonly in the pl.

Arm, v. t.
[imp. & p. p. Armed; p. pr. & vb. n. Arming.] Etym: [OE. armen, F. armer, fr. L. armare, fr. arma, pl., arms. See arms.]

1 To take by the arm; to take up in one’s arms. [Obs.] And make him with our pikes and partisans A grave: come, arm him. Shak. Arm your prize; I know you will not lose him. Two N. Kins.
2 To furnish with arms or limbs. [R.] His shoulders broad and strong, Armed long and round. Beau. & Fl.
3 To furnish or equip with weapons of offense or defense; as, to arm soldiers; to arm the country. Abram . . . armed his trained servants. Gen. xiv. 14.
4 To cover or furnish with a plate, or with whatever will add strength, force, security, or efficiency; as, to arm the hit of a sword; to arm a hook in angling.
5 Fig.: To furnish with means of defense; to prepare for resistance; to fortify, in a moral sense. Arm yourselves . . . with the same mind. 1 Pet. iv. 1. To arm a magnet, to fit it with an armature.

Arm, v. i.
To provide one’s self with arms, weapons, or means of attack or resistance; to take arms. ” ‘Tis time to arm.” Shak.