"Most coriaceous" refers to something that is tough, leathery, or hard to penetrate. In contrast, the antonyms for this word would be something that is soft, tender, delicate or easily penetrated. The opposite of coriaceous can be fragile, flimsy, weak, or thin. Additionally, antonyms could include words like pliable or malleable, which suggest flexibility or yielding. Sapless or insubstantial can also be antonyms for coriaceous, implying a lack of strength or substance. Whatever the opposite of coriaceous may be, it is a term that describes something markedly different from its rough, durable counterpart.