Green’s Dictionary of Slang

a general intensifier, excessively, immensely, greatly.

a.1743		J. Hervey Memoirs Court of George II n.p.: [Mention is made of Queen Caroline’s indignation at the infliction of] a thundering long sermon [F&H].
1828		‘A Flat Enlightened’ Life in the West I 41: ‘The Marquis of Meadowdale, a thundering rich old buck’.
1835		D. Crockett Col. Crockett’s Tour to North and Down East 61: I reckon old King George thought they were thundering fine children.
1853		Dickens Bleak House (1991) 296: I was a thundering bad son, that’s the long and short of it.
1855		T. Haliburton Nature and Human Nature I 203: A thunderin’ long word, ain’t it.
1867		J. Greenwood Unsentimental Journeys 99: What do I warnt? What do you think I warnt? thunderin’ well you knows I warnt everythink.
1870		E. Greey Queen’s Sailors III 4: I’ve heered a slave in the Brayzills say [...] that slavery was a thundering good thing for everybody wot hadn’t got no money.
1880		Henley & Stevenson Deacon Brodie II tab.IV viii: Don’t you get coming the nob over me, Mr. Deacon Brodie, or I’ll smash you [...] Ay will I. If I thundering well swing for it.
1890		‘Rolf Boldrewood’ Colonial Reformer II 151: Certainly it’s thundering hot.
1900		Sydney Sportsman (Surry Hills, NSW) 31 Oct. 1/3: Some of the people The Sportsman has punched the hardest are thundering decent fellows away from the [racing] game.
1905		Marvel III:54 8: It would be thundering unlikely.
1935		A. Christie Three Act Tragedy (1964) 28: You’re a thundering good chap, Charles.
1942		A. Christie Body in the Library (1959) 103: He’s a thundering good actor.
1956		‘Charles Raven’ Und. Nights 74: Fraser was [...] a partner in his own firm which was a well-known one and did a thundering good business.
1960		H.E. Bates When the Green Woods Laugh (1985) 252: ‘Thundering hot still,’ Pop said.