Green’s Dictionary of Slang

overwhelming, extreme; esp. in phr. crashing bore, an extremely boring person; thus crashingly adv.

1858		O.W. Holmes Autocrat of the Breakfast Table 52: The prisoner sniffed, and with a crashing sneeze, / Off his head tumbled.
1941		P.C. Wren Odd – But Even So 19: I’ve such a crashing headache.
1965		P. Highsmith Suspension of Mercy (1971) 90: Sir Quentin Ogilvie, K.C.M.G., G.C.B. – which stands for Grand Crashing Bore.
1972		(con. 1916) G. Swarthout Tin Lizzie Troop (1978) 124: His sobriety would turn him, in later years, into a crashing bore.
1983		(con. 1950s) D. Nobbs Second From Last in the Sack Race 268: Cecil E. Jenkinson, licensed to be a crashing bore and have a son who was as thick as pig shit.
1999		Indep. Rev. 3 Aug. 9: Some of our recent female Cleopatras [...] have made the queen’s capricious volte-faces look crashingly pre-meditated.