Green’s Dictionary of Slang

1. (also soft salve, olive oil) flattery, emollient speech.

1821		Military Reg. 14 Mar. 38: I'm little given to compliments I own, They're but the bubbles of our riper days, Which by the breath of flattery are blown From soft-soap froth, to pass for honest praise [OED].
1827		J. Wight More Mornings in Bow St. 226: But if Samuel is subject to fits of sauciness, we would recommend him to carry as little soft soap about him in future.
1833		Satirist (London) 17 Feb. 476/3: ‘No objection,’ said the leary member, ‘to try to get the duty taken one kind of soap. [...] I mean soft soap’.
1840		W.G. Simms Border Beagles (1855) 330: The fellow’s no fool when he ain’t flattered—it’s soft soap only that turns his head.
1840		Flash Mirror 20: Miss Cafooselem [...] stowing all chaff and soft soap.
c.1850		F.M. Whitcher Widow Bedott Papers (1883) 103: I dident know afore that you ever used any soft soap.
1861		T. Hughes Tom Brown at Oxford (1880) 358: He and I are great chums, and a little soft-soap will go a long way with him.
1877		J. Greenwood Dick Temple III 202: She’d be down on her knees and begging me to take her [...] if I gave her soft-soap enough.
1887		G. Devol Forty Years a Gambler 112: Molasses will catch more suckers than soft soap.
1895		Crowe Aus. Sl. Dict. 78: Soft Soap, flattery or ironical praise.
1900		Boy’s Own Paper 1 Dec. 132: He evidently wanted all soft soap, and didn’t like our finding fault.
1910		‘O. Henry’ ‘Friendly Call’ in Rolling Stones (1913) 114: ‘I’ve tried all sorts of ways.’ [...] ‘Tried soft soap?’.
1916		J. Buchan Greenmantle (1930) 294: I used to nod my head and get enthusiastic about these stunts, but it was all soft soap.
1929		‘Ellery Queen’ Roman Hat Mystery 220: You’re not going to get anywhere with this soft soap.
1934		H. Miller Tropic of Cancer (1963) 301: Fillmore said something to me [...] about giving her a little soft soap.
1944		J. Cary Horse’s Mouth (1948) 244: By getting a masterpiece that’s going to be worth ten thousand pounds for a couple of hundred dirty money and two of soft soap.
1955		B. Schulberg On the Waterfront (1964) 14: I look to con him out of my way with some soft soap.
1968		K. Brasselle Cannibals 45: They’re not suckers for the soft salve. [Ibid.] 146: ‘She said she was dying to meet you,’ I lied. ‘Spare me the olive oil.’.
1975		L. Rosten Dear ‘Herm’ 148: The Old Coot pores on the soft soap.
1996		R. Rendell Keys to the Street 134: Maybe she was running short of cash [...] and thought soft soap might secure her a discount.

2. (US) nonsense.

1834

Ely’s Hawk & Buzzard (NY) 22 Mar. 1/2: He will (if his brain continues to produce so much soft soap [...] ) soon become a powerful opposition to some of our famous soap-boilers.

3. attrib. use of sense 1.

1895		Truth (Sydney) 13 Jan. 4/6: These soft-soap-and-sawder gentry had a grand opportunity the other day, when they sped the parting guest.
1933		‘Goat’ Laven Rough Stuff 86: I fell in with the grifter’s plan to become a soft-soap con-man.
1971		F.J. Hardy Outcasts of Foolgarah (1975) 191: [He] declined saying he didn’t drink because he was a christian, a soft soap tell-them-what-they-want-to-know trick he’d learned when dealing with missionaries.

4. kindness.

1930		M. Fiaschetti You Gotta Be Rough 46: Kindness goes a long way [...] and you’d be surprised the way the soft soap will make the boys come clean.
1955		D. Niland Shiralee 207: They’d play around like poofters, with the kid gloves and the soft soap.
1963		G. Lambert Inside Daisy Clover (1966) 8: The Dealer told him to lay off the soft soap.
2000		S. Maloney Big Ask 199: ‘Joys of single fatherhood, right?’ ‘Save the soft soap,’ I snarled.
2007		S. Maloney Sucked In 191: Bishop gave an understanding nod. He opened his mouth. Before he could apply the soft soap, I changed tack again.