Unheardof, meaning unknown or unprecedented, has several antonyms. Familiar, common, and ordinary are all opposite in meaning to unheardof. Something that is familiar is well-known or recognized, while something that is common is widespread or of everyday usage. Ordinary, on the other hand, refers to something that is unremarkable or lacking distinction. Other antonyms for unheardof may include well-known, recognized, common, usual, and conventional. In short, there are many words that can be used as antonyms for unheardof, all of which help to convey a sense of familiarity, recognition, and normalcy.