Arm in arm

What is Arm in arm?

Arm in arm definition and meaning on Dictionary terms:
the upper limb of the human body, especially the part extending from the shoulder to the wrist.
the upper limb from the shoulder to the elbow.
the forelimb of any vertebrate.
some part of an organism like or likened to an arm.
any armlike part or attachment, as the tone arm of a phonograph.
a covering for the arm, especially a sleeve of a garment: the arm of a coat.

an administrative or operational branch of an organization: A special arm of the government will investigate.
Nautical. any of the curved or bent pieces of an anchor, terminating in the flukes.
an armrest.
an inlet or cove: an arm of the sea.
a combat branch of the military service, as the infantry, cavalry, or field artillery.
power; might; strength; authority: the long arm of the law.
Typography. either of the extensions to the right of the vertical line of a K or upward from the vertical stem of a Y.