An Etymological Dictionary of Astronomy and Astrophysics

1. Each of the upper limbs of the human body, especially the part between the → shoulder and the → wrist.
   

2. The upper limb from the shoulder to the → elbow.
   

3. A slender part of a structure projecting from a main part, such as a → spiral arm. → Orion Arm;
   → Perseus Arm; → Scutum-Crux Arm.

Etymology (EN): M.E. arm, from O.E. earm “arm,” from P.Gmc. *armaz (cf. M.Du., Ger. Arm, O.N. armr, O.Fris. erm), from PIE base *ar- “to fit, join;” cf.
Mod.Pers. arm, as below.

Etymology (PE): 1) Ârm (Dehxodâ, Steingass) “arm, from the elbow to the shoulder;” Av. arma-, arəmo- “arm;” cf. Ossetic ärm “hand;” Armenian armuku “elbow;” Skt. irma- “arm;”
Gk. arthron “a joint;” L. armus “shoulder;” cognate with E. arm, as above.

2. Bâzu, → upper arm.