rigor n.

Proto-Indo-European root meaning "stretch; be stretched; be stiff" 

It forms all or part of: reach; rigid; rigidity; rigor.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit rjyati "he stretches himself," riag "torture" (by racking); Greek oregein "to reach, extend;" Latin rigidus "hard, stiff, rough, severe," rigēre "be stiff;" Lithuanian raižytis "to stretch oneself;" Old Irish ringid "torture," rigim "I stretch;" Middle High German ric "band, string;" Old High German reihhon, Old English ræcan, ræcean "to reach, achieve," on notion of "to stretch out." 

1620s, "stiffness, inflexibility," especially in mechanics, "resistance to change of form;" 1620s, from Latin rigiditas "stiffness," from rigidus "hard, stiff, rough, severe" (see rigid). By 1650s as "strictness, severity," but rigidity tends to be used of physical stiffness, while rigor is more active or moral. Rigidness (1640s) "perhaps holds a middle position" [Century Dictionary].

Rigidity is directly opposed to flexibility, and only indirectly to malleability and ductility, which depend chiefly on relations between the tenacity, the rigidity, and the limit of elasticity. [Century Dictionary]