Etymology
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venial (adj.)
c. 1300, "pardonable," from Old French venial "pardonable, excusable" (13c.) and directly from Late Latin venialis "pardonable," from Latin venia "forgiveness, indulgence, pardon, favor," from PIE *wen-ya- "sexual love, desire," suffixed form of root *wen- (1) "to desire, strive for." Related: Venially.
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*wen- (1)
Proto-Indo-European root meaning "to desire, strive for."

It forms all or part of: vanadium; Vanir; venerate; veneration; venerable; venereal; venery (n.1) "pursuit of sexual pleasure;" venery (n.2) "hunting, the sports of the chase;" venial; venison; venom; Venus; wean; ween; Wend "Slavic people of eastern Germany;" win; winsome; wish; wont; wynn.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit veti "follows after," vanas- "desire," vanati "desires, loves, wins;" Avestan vanaiti "he wishes, is victorious;" Latin venerari "to worship," venus "love, sexual desire; loveliness, beauty;" Old English wynn "joy," wunian "to dwell," wenian "to accustom, train, wean," wyscan "to wish."
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mortal (adj.)

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.

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