The word "most foundationless" implies a lack of basis or support. Some antonyms for this phrase could be "stable," "solid," "secure," or "substantial." These terms suggest a strong foundation or basis for something, whether it be an idea, a building, or a relationship. Another antonym for "most foundationless" could be "rooted," signifying a deep connection with something or a sense of belonging. As opposed to being adrift or unanchored, being rooted implies a strong sense of purpose and direction. Overall, antonyms for "most foundationless" convey the opposite qualities of instability, insecurity, and uncertainty.