Antonyms for the word "most coterminous" would be phrases like "least adjacent" or "furthest apart." The term coterminous means two things sharing a boundary or having the same boundaries, so antonyms would refer to the exact opposite, meaning two things that do not share a boundary or have completely different boundaries. Other antonyms could include "disconnected," "separated," or "distant." Using antonyms can be helpful in expressing differences between things or ideas and can also add depth to writing by demonstrating a wide range of vocabulary.