Origin and history of temeration

early 15c., temerite, "extreme venturesomeness, rashness, recklessness," from Old French temerite (14c.) and directly from Latin temeritatem (nominative temeritas) "blind chance, accident; rashness, indiscretion, foolhardiness," from temere "by chance, at random; indiscreetly, rashly, recklessly;" probably, etymologically, "blindly."

This is reconstructed to be from a PIE *temos- "darkness," 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).