Soft soap, soft solder (pron. saw-der), “wiping down” with winning words. Punch expressively calls it “the milk of human kindness churned into butter.” (Anglo-Saxon, butere or butyre, Latin, butyrum, Greek, boutyron, i.e. bou-turos, cow-cheese, as distinguished from goat- or ewe-butter.)
`Be thou fed,' will not feed a hungry man. Mere words will not find salt to our porridge, or butter to our parsnips. Fine words, says our homely old proverb, butter no parsnips.
He looks as if butter would not melt in his mouth. He looks like a dolt. He looks quite harmless and expressly made to be played upon. Yet beware, and “touch not a cat but a glove.”
“She smiles and languishes, you'd think that butter would not melt in her mouth.” —Thackeray: Pendennis, ix.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894