The term "foldable" refers to an object or material that can be bent, creased, or compressed to occupy less space. Its antonyms, therefore, would be words that signify the opposite meaning. Some possible antonyms for "foldable" include rigid, inflexible, solid, immovable, and unbending. These words imply that an object is incapable of being bent or compressed. Another set of antonyms for "foldable" would be words that suggest massiveness or bulkiness, such as bulky, cumbersome, unwieldy, or hefty. Altogether, these antonyms highlight the versatility and adaptability of foldable materials while underscoring the limitations and drawbacks of objects that cannot be folded.