An Etymological Dictionary of Astronomy and Astrophysics

1. Severely exact or accurate; precise.
   

2. Logic, Math.: Logically valid. See also: → accurate, → exact, → precise.

Etymology (EN): M.E., from M.L. rigorosus, from rigor “stiffness, rigor,” from rigere “to be stiff.”

Etymology (PE): Farsaxt, from far- intensive prefix “much, abundant; elegantly” (Mid.Pers. fra- “forward, before; much; around;”
O.Pers. fra- “forward, forth;” Av. frā, fərā-, fra- “forward, forth; excessive;” cf. Skt. prá- “before; forward, in fron;” Gk. pro “before, in front of;” L. pro “on behalf of, in place of, before, for;” PIE *pro-)

• saxt “hard, strong, firm, secure, solid, vehement, intense” (Mid.Pers. saxt “hard, strong, severe;” Av. sak- “to understand or know a thing, to mark;” cf. Skt. śakta- “able, strong,” śaknoti “he is strong,” śiksati “he learns”).