An Etymological Dictionary of Astronomy and Astrophysics

1. General: Stiff or unyielding; inflexible, strict, severe.
   
2. Physics: → rigid body; → rigidity.

Etymology (EN): From L. rigidus “hard, stiff, rough,” from rigere “to be stiff;” related to L. frigus “cold,” Gk. rhigos “frost, cold.”

Etymology (PE): Saxtpây, from saxt, → hard, + pây present stem of pâyidan “to stand firm, to be constant, steady, fixed;” Mid.Pers. pâyitan, pâtan, pây- “to protect; wait, stand,” Sogdian p’y “to protect, watch over;” O.Pers./Av. pâ(y)- “to protect, keep” pâtar- “protector, watcher,” cf. Skt. pâ- “to protect, keep,” pâti “protects;” Gk. poimen “shepherd,” poma “lid, cover;” L. pastor “shepherd,” panis “bread;” PIE base *pa- “to protect, guard, pasture, feed.”