Most membranous generally refers to something that has a thin, flexible, and usually transparent layer. Antonyms for this phrase can be discussed in multiple ways. For instance, the opposite of most membranous could be described as thick, rigid, and opaque. Another interpretation of the antonyms for most membranous could be solid, structured, and substantial. Alternatively, the phrase could be interpreted in a hierarchical sense, where the antonyms could be interpreted as least membranous or spineless, indicating something that lacks structure, thickness, or rigidity. These antonyms can add depth and variety to the description of an object, organism, or structure, depending on the context and usage of the phrase.