Green’s Dictionary of Slang

damn! I’m off.

1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) n.p.: P.P.C. An inscription on the visiting cards of our modern fine gentlemen, signifying that they have called pour prendre congé, i.e. ‘to take leave.’ This has of late been ridiculed by cards inscribed D.I.O. i.e. ‘Damme, I’m off’.
1811		Lex. Balatronicum.
1812		N.-Y. Eve. Post 22 Aug. 3/2: [Like] the wag who stole away from the company he was in, leaving a paper marked Dio, that is, ‘Damme, I am off.’.
1821		R. Waln Hermit in America on Visit to Phila. 2nd ser. 26: ‘D.I.O!’ — The blood made his leg [bowed] and staggered out.
1827		W.T. Moncrieff Bashful Man II ii: Then D.I.O. my boy. I’ll be sure to be back in time for dinner.
1859		Matsell Vocabulum.
1874		‘Old Calabar’ Won in a Canter III 55: ‘My time will be up at the cottage the week after next [...] and then, D.I.O.’.