This is a great way of saying that two things are so similar that there is no significant difference between them. I'm sure there are many more and thought this might make a great community wiki. How did the phrase/idiom come about and is it more prevalent in one english-speaking region than another?
This site says that it comes from a 1728 play. To the extent that it's known nowadays, though (it's not really prevalent anywhere, as far as I know), it's due to its use in Lewis Carroll's Alice in Wonderland.
Sometimes an idiom seems like a cliche, where there is a vague meaning. When their history is seen, they are often easier to understand, and any doubt about their meaning can be put into context. It's always better to know what we mean when we use them, and have a better idea of what others are likely to think they mean. Knowing their history can color in some of the blank spaces.
For "much of muchness" - Similar - difficult to distinguish.
Credit : Pokket