c. 1300, "heavily," also "solidly," from sad + -ly (2). The main modern meaning "sorrowfully" begins by mid-14c.
Entries linking to sadly
sad adj.
Old English sæd "sated, full, having had one's fill (of food, drink, fighting, etc.), weary of," from Proto-Germanic *sathaz (source also of Old Norse saðr, Middle Dutch sat, Dutch zad, Old High German sat, German satt, Gothic saþs "satiated, sated, full"), from PIE *seto-, from root *sa- "to satisfy." Related: Sadder; saddest.
In Middle English and into early Modern English the prevailing senses were "firmly established, set; hard, rigid, firm; sober, serious; orderly and regular," but these are obsolete except in dialect. The sense development seems to have been via the notion of "heavy, ponderous" (i.e. "full" mentally or physically), thus "weary, tired of." By c. 1300 the main modern sense of "unhappy, sorrowful, melancholy, mournful" is evident. An alternative course would be through the common Middle English sense of "steadfast, firmly established, fixed" (as in sad-ware "tough pewter vessels") and "serious" to "grave." In the main modern sense, it replaced Old English unrot, negative of rot "cheerful, glad."
By mid-14c. as "expressing or marked by sorrow or melancholy." The meaning "very bad, wicked" is from 1690s, sometimes in jocular use. Slang sense of "inferior, pathetic" is from 1899; sad sack is 1920s, popularized by World War II armed forces (specifically by cartoon character invented by Sgt. George Baker, 1942, and published in U.S. Armed Forces magazine "Yank"), probably a euphemistic shortening of the common military slang phrase sad sack of shit.
-ly 2
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.