rigor mortis

late 14c., "deadly, destructive to life; causing or threatening death" (of illness, poisons, wounds, etc.); also, of persons or the body, "doomed to die, subject to death;" from Old French mortel "destined to die; deserving of death" and directly from Latin mortalis "subject to death, mortal, of a mortal, human," from mors (genitive mortis) "death."

This is reconstructed to be from PIE *mr-o- "to die," *mr-to- "dead," *mr-ti- "death," all from PIE root *mer- "to rub away, harm" (also "to die" and forming words referring to death and to beings subject to death). The most widespread Indo-European root for "to die," it forms the common word for it except in Greek and Germanic.

"Subject to death," hence "human, of or pertaining to humans" (early 15c.). Also from late 14c. as "implacable, to be satisfied only by death" (of hatreds, enemies, etc.). Meaning "extreme, very great" is from late 14c. A mortal sin (early 15c., opposed to venial) is one that incurs the penalty of spiritual death.

late 14c., rigour, "harshness, severity in dealing with persons; force; cruelty," from Old French rigor "strength, hardness" (13c., Modern French rigueur), from Latin rigorem (nominative rigor) "numbness, stiffness, hardness, firmness; roughness, rudeness," from rigēre "be stiff" (from PIE root *reig- "stretch; be stretched; be stiff").

Also, in medieval medicine, "a sudden chill" (c. 1400). From early 15c. as "exactness, strictness without indulgence" (of discipline, the law, etc.). Compare rigidity. Rigorism "rigidity in principles or practice" (originally religious) is from 1704. Rigidist is by 1716.