The phrase "unutterable sins" refers to immoral or unethical actions that are so unspeakable or unimaginable that they cannot be expressed verbally. Synonyms for this phrase may include heinous crimes, despicable deeds, abominable acts, atrocious offenses, unspeakable atrocities, unforgivable transgressions, and abhorrent wrongdoing. These terms all concur on the extreme heinousness and depravity of the acts or deeds indicated by the phrase "unutterable sins". They convey a sense of moral outrage and condemnation towards those who have committed such acts. The synonyms highlight a range of actions that might include rape, murder, genocide, and terrorism among others.