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.
Related Words for but gooddecidedly, eminently, deeply, tremendously, exceptionally, extraordinarily, profoundly, terribly, exceedingly, so, greatly, vastly, extremely, remarkably, really, strikingly, hugely, supremely, immensely, awful
Examples from the Web for but good
Historical Examples of but good
Till now one with sudden hiss: "But-good Christ-just look-why, the roof's leaning—!"The Lord of the Sea
M. P. Shiel