Similar - difficult to distinguish.

This sounds as typical of Shakespeare's coinages. In fact it first appears in the play The Provok'd Husband (1728), which was a collaboration between John Vanbrugh and Colley Cibber:

Man: I hope.., you and your good Woman agree still. J. Moody: Ay! ay! much of a Muchness.

Muchness means physical magnitude or largeness and is derived from the earlier word mickleness.

Much of a muchness has remained as part of the language since Vanbrugh's day, but has never been commonplace. It is rather and odd phrase on the face of it as, in literal terms, it just means 'of a similar quality of being much'. Lewis Carroll picked up on that oddness when, in Alice's Adventures in Wonderland (1865), he had the Dormouse ask Alice "That begins with an M, such as... muchness - you know you say things are 'much of a muchness' - did you ever see... a drawing of a muchness?"