Green’s Dictionary of Slang

an intensifying adj. dependent for pos./neg. implication on the noun to which it is attached; lit. the use of a dash when printing damned adj.

1878		J.H. Nicholson ‘Bunkum in Parvo’ in Opal Fever 113: If any dashed new chum should set upon you, I’ll show you, you duffer, what I would do.
1888		‘Rolf Boldrewood’ Robbery Under Arms (1922) 165: We’ve been dashed lucky so far.
1901		‘Miles Franklin’ My Brilliant Career 150: The boss is so dashed partickler too.
1902		Harrington & LeBrunn ‘Charley was a Good, Good Boy’ 🎵 As the 'bookies' all asserted with unnecessary force / C Charley was a dashed, good boy.
1903		J. Furphy Such is Life 224: I’m too dash lazy.
1918		C.J. Dennis ‘Half A Man’ in Chisholm (1951) 103: ‘I wash me ’ands uv ’im,’ I tells ’em, straight. / ‘You women can do wot yeh dash well like.’.
1922		E. Raymond Tell England (1965) 33: You never let me, sir – dashed unfair.
1934		D.L. Sayers Nine Tailors (1984) 50: I shouldn’t care for the sluice-keeper’s job – dashed lonely, I should think.
1942		Whizzbang Comics 66: That’s dashed hard lines on Podger and the rest.
1960		Wodehouse Jeeves in the Offing 45: Dashed difficult.
1976		L. Dawson The Spy Who Came... 83: ‘I’m dashed glad you disobeyed my order’.
1981		S. Berkoff Decadence in Decadence and Other Plays (1985) 23: Dashed good form.