The term 'most hairline' refers to something that is very thin or narrow, and has no obvious or visible thickness. The antonyms to this term can be 'thick', 'broad', 'wide', 'ample', 'hefty', 'fat', 'plump', 'brawny' or 'bulky'. These terms signify a lack of slimness, a fuller or broader form, and an overall appearance of being sturdy or massive. Using these antonyms, one can describe objects, people or places that are the opposite of being 'most hairline', and thus provide a bigger and more comprehensive understanding of the world around us.