It's possible that it's:
* A misspelling: Perhaps you meant a similar-sounding Latin word like "tenebris" (meaning "darkness"), "tenebris" (meaning "shadows"), or "tenebrous" (meaning "dark or shadowy").
* A derivative: It could be a word derived from Latin, but not a word that would be found in a standard Latin dictionary.
* A fictional word: It might be a term used in a fictional context, like a fantasy novel or game.
If you can provide more context about where you encountered "tenebo," I might be able to help you figure out what it means.