Emphatically, thoroughly, as in Ruth decided to clean up the whole yard but good. The word but in this colloquialism functions as an intensive. Also see and how.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
How to use but good in a sentence
Till now one with sudden hiss: "But-good Christ-just look-why, the roof's leaning—!"The Lord of the Sea | M. P. Shiel