An Etymological Dictionary of Astronomy and Astrophysics

A low-density region separating the spiral arms of a galaxy.

Etymology (EN): Interarm, from → inter- + arm “body part,” 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;”
Mod.Pers. arm “arm, from the elbow to the shoulder;” Av. arma-, arəmo- “arm;” Skt. irma- “arm;”
Gk. arthron “a joint,” L. armus “shoulder;” → region.

Etymology (PE): Andarbâzu, from andar-, → inter-, + bâzu “arm,” from
Mid.Pers. bâzûk “arm;” Av. bāzu- “arm;” cf. Skt. bāhu- “arm, forearm;” Gk. pechys “forearm, arm, ell;”
O.H.G. buog “shoulder;” Ger. Bug “shoulder;” Du. boeg; O.E. bôg, bôh “shoulder, bough;” E. bough " a branch of a tree;" PIE *bhaghu- “arm”); nahiyé, → region. Andararm, from andar-, → inter-, + arm, as above.