The word "tenebrous," an adjective (pronounced ten'-uh-brus) has origins in Middle English and its first documented use was in the 15th century in England and/or France. It is derived from the Latin words tenebrae and tenebrosus. There are several different yet similar definitions of the word.
- In one form it means "cut off from light."
- Another form is "difficult to understand."
- A third version is "a state of gloom or shadows"
Other forms of the word include tenebrosity (noun), tenebrousness (noun), and tenebriousness (noun). Synonyms include dark, gloomy, murky, obscure, somber, black, shadowy, lightless.
To build on the above answer, the word "tenebrous" is definitely a word imported into English from French. The Online Etymology Dictionary (see the link below) gives the date "early 15c," which means that the earliest documented use of the word "tenebrous" in English is from the early 15th century. The word may have entered the English language earlier, of course.
The French word used today -- ténébreux, -euse -- means pretty much the same thing: shadowy, obscure, mysterious, etc.
In a Simple word : DARK