severely adv.

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.

common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).

Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.