Very; exceedingly. A vulgar word used with pretty much the same latitude as the English devilish.
I was told that Fanenil Hall was called the "cradle of liberty." I reckon old King George thought they were thundering fine children that were rocked in it, and a good many of them.--Crockett, Tour down East, p. 61.
If a chap only comes from the North, and has got a crop of hair and whiskers, and a coat different from everybody else, and a thunderin' great big gold chain about his neck, he's the poplerest man among the ladies.--Maj. Jones's Courtship, p. 82.