The word "rock-ribbed" is used to describe something that is extremely firm or unyielding. However, there are a number of antonyms that can be used to describe something that is soft, pliable or flexible. For example, a synonym for "rock-ribbed" might be "unbending" or "rigid," while antonyms could include "yielding," "flexible," or "plastic." Other antonyms for "rock-ribbed" could include "malleable," "supple," or "pliant." Depending on the context in which the word is being used, there are a wide variety of words that can be used to describe something that is not "rock-ribbed.