If something's larrupin' good, it's spankin' good or thumpin' good. It comes from the word larrup, a verb meaning "to beat or thrash." This is part of a complete episode.
My mother was from Missouri. She used the word larrupin to describe especially delicious food (after tasting it). (A couple generations prior to hers immigrated from England, so I thought there might be a chance it had been carried over).
I am having trouble understanding how the meaning is related to the meaning of the original word meaning thrashing or beating.
I have heard of "brand spanking new", but I have never heard of spankin' good or thumpin' good.
One source said larrupin is a word from a dialect used in the western US, which also puzzled me, as the limited range doesn't include the part of the country I grew up in (Missouri) and heard it used. I have lived in Colorado for 45 years and have never heard it used here. Everyone looks at me cross-eyed when I use it.
(I have also seen larrupin defined as too sweet, cloying, which is surprising as it is a derogatory term rather than complimentary).