severity (n.)

1540s, "rigorous in condemnation or punishment," from French severe (12c., Modern French sévère) or directly from Latin severus "serious, grave, strict, austere," which is of uncertain origin, but de Vaan supports the theory (also in Watkins) that it probably is a suffixed form PIE root *segh- "to have, hold," on the notion of "steadfastness, toughness."

It is attested by 1560s of looks and demeanor, of law or punishment ("unsparing"), also "extremely strict in matters of conduct or self-discipline." It is attested by 1660s with reference to styles or tastes, "chaste, restrained, shunning florid ornament." On the notion of "sharp, distressing, violent" it is attested by 1670s in reference to weather or winter, by 1725 of illness or disease, by 1742 of pain and suffering.