An Etymological Dictionary of Astronomy and Astrophysics

1. Mechanics: The rotation of a body about an axis through the body.
   To cause to turn around rapidly, as on an axis. To revolve or rotate rapidly,
   
2. Quantum mechanics: See → spin quantum number; → spin angular momentum.

Etymology (EN): M.E. spinnen; O.E. spinnan “to draw out and twist fibers into thread” (cf. O.N., O.Fris. spinna, Dan. spinde, Du. spinnen, O.H.G. spinnan, Ger. spinnen); cognate with Pers. tan-, tanidan
“to spin, twist, weave” (Mid.Pers. tanitan; Av. tan- to stretch, extend;" cf. Skt. tan- to stretch, extend;" tanoti “stretches,” tántra- “warp; essence, main point;” Gk. teinein “to stretch, pull tight;” L. tendere “to stretch;”
Lith. tiñklas “net, fishing net, snare,” Latv. tikls “net;” PIE base *ten- “to stretch”).

Etymology (PE): Espin, loan from E., as above.