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?
Learn English – How did the phrase “much of a muchness” come to be
idioms
Best Answer
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.