Old English cearful "mournful, sad," also "full of care or woe; anxious; full of concern" (for someone or something), thus "applying attention, painstaking, circumspect" (late Old English), the main modern sense; from care (n.) + -ful. In Middle English also "miserable, unfortunate," of persons or things; "causing fear, frightening, terrible." Careful-bed (early 14c.) was "sick-bed;" careful-day (c. 1200) was "judgment day."
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).
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Harper, D. (n.d.). Etymology of carefulness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/carefulness