Middle English recheles, from Old English receleas "careless, thoughtless, heedless," earlier reccileas, literally "not recking (of consequences)" from *rece, recce "care, heed," from reccan "to care" (see reck (v.)) + -less. The same affixed form is in German ruchlos, Dutch roekeloos "wicked."
The root verb reck (Old English reccan) is passing into obscurity; the range of Middle English spellings might reflect uncertainty even then about it, e.g. rechiles, retcheles, recelease, richeles, regeles, reccles, rakeles.
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.
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Harper, D. (n.d.). Etymology of recklessly. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/recklessly