arm has 9 distinct synonyms spread across 5 of its 8 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is disarm.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a human limb; technically the part of the superior limb between the shoulder and the elbow but commonly used to refer to the whole superior limb
any projection that is thought to resemble a human arm
“the arm of the record player”
any instrument or instrumentality used in fighting or hunting
“he was licensed to carry a weapon”
the part of an armchair or sofa that supports the elbow and forearm of a seated person
a division of some larger or more complex organization
“a branch of Congress”
the part of a garment that is attached at the armhole and that provides a cloth covering for the arm
Synonyms, antonyms and the broader and narrower relations for arm come from Princeton University’s WordNet. Full sources and licences.