"rash, reckless," 1530s, from Latin temerarius "rash, heedless, thoughtless, indiscreet," from temere "blindly, rashly, by chance" (see temerity). Related: Temerariously; temerariousness.
Entries linking to temerarious
temerity n.
"extreme venturesomeness, rashness, recklessness," late 14c., from Latin temeritatem (nominative temeritas) "blind chance, accident; rashness, indiscretion, foolhardiness," from temere "by chance, at random; indiscreetly, rashly, recklessly;" probably, etymologically, "blindly," from PIE root *temsro- "dark" (adj.), source also of Sanskrit tamisra- "dark night," tamsrah "dark;" Avestan temah "darkness;" Middle Persian tar "darkness," tarig "dark;" Lithuanian tamsa "darkness," tamsus "dark;" Old Church Slavonic tima "darkness;" Old High German dinstar "dark," demar "twilight;" Old Irish temel "darkness."
The connecting notion would be "blindly, in darkness," hence "without foreseeing." Compare Latin tenebrio "dishonest person," apparently "person who operates in darkness" (see tenebrous).