Heard in Missouri: "but good!"

[David Donnell]

Question about an expression I just heard: "but good!"

I just returned from St Louis where I was visiting my 88 yr old
mother. (Born & raised in southeast Missouri, near Cape Girardeau.)

Addressing me and her friend/contemporary, my mom related a
conversation about someone's 16 yr old daughter: the girl had stayed
out until 3 a.m., sans permission, with her cell phone turned off...
Very worrisome to the parents, natch.

My mom's friend then asked if the girl would be punished. My mom
responded, "I asked [the father] if she was grounded...he said BUT
GOOD!"

The expression never registered with me before, although it sounded
familiar rolling off her tongue; it apparently means something like
"and how!"

Is anyone familiar with this critter?

(Hard to Google such a ubiquitous 2-word combo.)

DD
Missouri-native in NYC

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