"Most wellnigh" is a phrase that conveys the idea of almost, nearly, or very nearly. Antonyms for this phrase would include terms that suggest a complete lack of proximity, such as distant, far, or remote. Other antonyms might indicate a sense of being different or opposite, such as dissimilar, distinct, or contrasted. Finally, some antonyms might suggest an idea of something that is not quite as close as "most wellnigh," but still approaches proximity, such as fairly or moderately. Ultimately, the choice of antonyms will depend on the specific context in which "most wellnigh" is used and the desired effect that the writer or speaker wishes to achieve.