comrade in arms

:  an intimate friend or associate :  companion

:  a fellow soldier

:  communist

:  a human upper limb

:  the part between the shoulder and the wrist

:  something like or corresponding to an arm: such as

:  to furnish or equip with weapons

:  to furnish with something that strengthens or protects

:  to equip or ready for action or operation

:  a means (such as a weapon) of offense or defense

:  firearm

:  a combat branch (as of an army)