You will almost never meet this word by itself. It lives inside one fixed phrase: 'much of a muchness', which means two or more things are so similar that choosing between them hardly matters. If a friend asks which of two restaurants is better and you say 'they're much of a muchness', you mean neither is clearly better — they are both about the same, and often you are gently suggesting both are only so-so. It is British and Australian rather than American, and it sounds a little old-fashioned and comfortable, like something an older relative would say. Americans would more likely say 'six of one, half a dozen of the other'. On its own, 'muchness' once simply meant greatness or a large amount, but that use died out centuries ago; a modern English speaker who saw 'the muchness of the crowd' would think you had invented it. So treat 'much of a muchness' as one single unit of meaning, the way you treat 'by and large' — learn the whole phrase, do not take it apart.

（只用在 much of a muchness）半斤八兩、差不多、沒什麼差別