much of a muchness
Synonym discussion
Sense: idiom/1
"Much of a muchness" is a distinctly informal British expression that emphasises not just similarity but the practical irrelevance of the differences between two or more items. "Six of one, half a dozen of the other" is a more conversational alternative focused on two choices being equally good or bad. Plain "the same" or "alike" are neutral and far more common across registers, but they lack the dismissive, 'it does not matter which' tone that "much of a muchness" carries. For example, saying "the two phone plans are much of a muchness" implies you could flip a coin; saying "the two phone plans are the same" states a factual identity.
Etymology
The noun 'muchness' dates back to the 1500s, where it meant 'greatness in quantity or degree'. The full phrase 'much of a muchness' first appeared in the early 1700s, notably in the writings of Jonathan Swift, and has since settled into an informal idiomatic expression meaning 'very similar, to the point where the difference hardly matters'.