Origin and history of calmly

late 14c., of the sea, "windless, without motion or agitation;" of a wind, "light, gentle," perhaps via Old French calme "tranquility, quiet," or directly from Old Italian calma "quiet, fair weather," which probably is from Late Latin cauma "heat of the mid-day sun" (in Italy, a time when everything rests and is still), from Greek kauma "heat" (especially of the sun), from kaiein "to burn" (see caustic). The spelling was influenced by Latin calere "to be hot." The figurative application to social or mental conditions, "free from agitation or passion," is from 1560s.

common adverbial suffix, forming, from adjectives, adverbs signifying "in a manner denoted by" the adjective; Middle English -li, from Old English -lice, from Proto-Germanic *-liko- (source also of Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko). See -ly (1). It is 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.