foolishness (n.)

early 14c., from fool (n.1) + -ish. Older adjectives in Middle English were fool (c. 1200); folly (c. 1300). Old English words for this were dysig, stunt, dol. Related: Foolishly; foolishness.

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).