nocuous

You are probably more familiar with the adjective innocuous, meaning "harmless," than with its antonymous relative nocuous. Both nocuous and innocuous have immediate Latin predecessors: nocuus and innocuus. (The latter combines nocuus with the negative prefix in-.) Both words can also be traced back to the Latin verb nocēre, meaning "to harm." Other nocēre descendants in English include the familiar innocent and the less familiar nocent, which means "harmful." Nuisance (which originally meant, and still can mean, "a harm or injury") is a more distant relative. Nocuous is one of the less common nocēre descendants, but it does turn up occasionally.