Green’s Dictionary of Slang

see safe as the bank below .

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1920

Dundee Eve. Teleg. 1 June 6/2: As safe as a church mouse was the phrase the painter [...] had used to describe [...] painting the flagpole.

1. extremely safe or safely.

1864		E. Yates Broken to Harness III 79: I have the means of doing that, as safe as houses.
1873		Sl. Dict. 195: ‘Safe as houses,’ an expression to satisfy a doubting person; ‘Oh! it’s as safe as HOUSES,’ i.e., perfectly safe, apparently in allusion to the paying character of house property as an investment. It is said the phrase originated when the railway bubbles began to burst, and when people began to turn their attention to the more ancient forms of speculation, which though slow were sure.
1885		Bulletin (Sydney) 11 Apr. 12/3: ‘Colonel, have your troops kept the provisions safe?’ […] ‘Safe as a house, Sir Gerald – they have them concealed about their persons!‘ Sir Gerald (aloud): ‘Noble fellows!’ (Aside): ‘The devil burst them!’.
1894		G.F. Northall Folk-Phrases of Four Counties 10: Safe as houses.
1901		E.W. Hornung Black Mask (1992) 229: If she were only that I should feel safe as houses.
1914		C. Holme Lonely Plough (1931) 164: You’re safe as houses, and you know it.
1929		J.B. Priestley Good Companions 467: She’s safe as houses here now – can’t lose.
1934		D.L. Sayers Nine Tailors (1984) 302: ‘My wife and children – are they safe?’ ‘Safe as houses, Will.’.
1941		M. Panter-Downes ‘Fin de Siécle’ in Wartime Stories (1999) 67: ‘Safe as houses’ was a maxim which meant nothing nowadays with the dust drifting round London.
1953		B. Behan Scarperer (1966) 42: Limey says the dough is as safe as houses.
1970		C. Brown Down All the Days 77: Safe as a house, Bridie.
1970		A. Sillitoe Start in Life (1979) 343: Nobody’ll trace you [...] you’ll be safe as houses and right as rain.
1981		J. Sullivan ‘The Russians are Coming’ Only Fools and Horses [TV script] If they started dropping the bomb on us right now we’d be safe as houses brother, safe as houses!
1992		I. Rankin Wolfman 146: Safe as bloody houses.

2. extremely sure; very likely.

1874		Hardy Far from the Madding Crowd in Works II (1912) 460: ‘He must come without fail, and wear his best clothes.’ ‘The clothes will floor us as safe as houses.’.
1904		Marvel 29 Oct. 31: Do it, and if you do you’ll get the sack as safe as houses.
1906		E. Pugh Spoilers 289: You’d be pinched an’ scragged as safe as ’ouses. At any rate, you’d be put in stir for the rest o’ your natural.

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1874		‘Old Calabar’ Won in a Canter II 307: ‘Do you think he-will pull through, Bullfinch?’ he hoarsely demands of that worthy. ‘Safe as ninepence, Sir’.
1895		C. Crowe Aus. Sl. Dict. 52: Ninepence,‘as right as ninepence,’ all right.