tenebrism n.

word-forming element making nouns implying a practice, system, doctrine, etc., from French -isme or directly from Latin -isma, -ismus (source also of Italian, Spanish -ismo, Dutch, German -ismus), from Greek -ismos, noun ending signifying the practice or teaching of a thing, from the stem of verbs in -izein, a verb-forming element denoting the doing of the noun or adjective to which it is attached. For distinction of use, see -ity. The related Greek suffix -isma(t)- affects some forms.

"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).