arm

The word 'arm' for the upper limb has a clear and well-documented Indo-European pedigree, but it con‌​‍​‌​‌​‌​‌​‌​‌​‌​‌​‌​‌​‍​‌​‌​‍​‍​‌​‍​‌​‍​‌​‍​‍​‍​‌​‌​‌​‍​‌​‌​‌ceals a surprising semantic origin: the arm was not named for its strength or its reach but for its joints. It comes from Old English 'earm,' from Proto-Germanic *armaz, from PIE *h₂er-mo-, a derivative of the root *h₂er- (to fit, to join, to articulate). The arm is, etymologically, 'the jointed thing' — the limb defined by its articulation at the shoulder and the elbow.

The PIE root *h₂er- (to fit, to join) is productive across the family. In Latin, it gave 'armus' (the shoulder joint, the upper arm — cognate with English 'arm'), 'artus' (a joint, a limb), 'articulus' (a small joint — source of 'article' and 'articulate'), and 'ars' (genitive 'artis,' skill, art — originally the fitting together of parts). In Greek, it appears in 'arthron' (ἄρθρον, a joint), source of 'arthritis' (joint inflammation). The connection between arms, joints, articles, art, and arthritis through a single PIE root meaning 'to fit together' is one of the more elegant etymological networks in the language.

It is essential to note that 'arm' (the body part) and 'arms' (weapons) are entirely different words that happen to have converged in English spelling. 'Arms' meaning weapons comes from Latin 'arma' (tools of war, equipment), through Old French 'armes.' The Latin word is of uncertain further etymology and is not related to *h₂er-. The coincidence is a false cognate — two unrelated words that look identical in modern English.

The compound vocabulary built on the body-part 'arm' is relatively modest: 'forearm' (the lower portion, from wrist to elbow), 'armpit' (from Old English 'earm' + 'pytt,' pit or hollow), 'armlet' (a bracelet for the upper arm), and 'armchair' (a chair with supports for the arms). The figurative 'arm' of the law or the 'arm' of a river (a branch) extends the sense of a projecting limb.

The Germanic cognates are regular: German 'Arm,' Dutch 'arm,' Swedish 'arm,' Danish 'arm,' Old Norse 'armr,' Gothic 'arms.' The word's phonological stability across Germanic — with almost no change from Proto-Germanic *armaz to the modern forms — is notable and reflects the high frequency and early acquisition of body-part vocabulary.

The related Latin word 'armus' specifically meant the shoulder or the uppermost part of the arm, a meaning preserved in veterinary terminology where the 'armus' refers to the shoulder region of a quadruped. The semantic range of 'arm' has been broader in Germanic, covering the entire limb from shoulder to hand, while Latin tended to distinguish more precisely between 'armus' (shoulder), 'bracchium' (upper arm, from Greek), and 'lacertus' (also upper arm).