foolhardy (adj.)youdaoicibaDictYouDict[foolhardy 词源字典]
also fool-hardy, mid-13c., folhardi, from fol "fool" (see fool (n.1) + hardi "bold" (see hardy) hence "foolishly brave, bold without judgement or moderation." Compare Old French fol hardi. Related: foolhardiness (mid-13c.); Middle English also had as a noun foolhardiment (mid-15c.).[foolhardy etymology, foolhardy origin, 英语词源]
hardy (adj.)youdaoicibaDictYouDict
c. 1200, "bold, daring, fearless," also "presumptuous, audacious," from Old French hardi "bold, brave, courageous; confident, presumptuous," from past participle of hardir "to harden, be or make bold," from Frankish *hardjan "to make hard" (cognates: Old Frisian herda, Old High German herten, Old Norse herða, Gothic gahardjan "make hard"), from Proto-Germanic *hardu- (see hard (adj.)). Sense influenced by English hard. Of plants, "able to survive in the open year-round," 1660s. Related: Hardily; hardiness. Hardhede "physical hardiness" is attested from early 15c.
temerity (n.)youdaoicibaDictYouDict
late 14c., from Latin temeritatem (nominative temeritas) "blind chance, accident; rashness, indiscretion, foolhardiness," from temere "by chance, at random; indiscreetly, rashly," related to tenebrae "darkness," from PIE root *teme- "dark" (cognates: Sanskrit tamas- "darkness," tamsrah "dark;" Avestan temah "darkness;" Lithuanian tamsa "darkness," tamsus "dark;" Old Church Slavonic tima "darkness;" Old High German dinstar "dark;" Old Irish temel "darkness"). The connecting notion is "blindly, without foreseeing."