There are several synonyms for the phrase "inexpressibly bad" that can be used to describe something exceptionally terrible or horrific. Such synonyms include "appalling," "atrocious," "abysmal," "catastrophic," "terrible," "ghastly," "gruesome," "horrific," "dreadful," "dire," "frightful," "heinous," "horrendous," "infernal," "monstrous," "nightmarish," "odious," "repugnant," "revolting," "shocking," "unbearable," and "unspeakable." Using these descriptive synonyms can help to convey the extent of the severity of something that is truly inexpressibly bad.