To that extent or degree, as in You decided to stay home? So much the better, for now we won't need a second car. This usage is always followed by a comparative adjective, such as better in the example. [Early 1200s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.