1
general::
Flattery, cajolery, as in She's only six but she's learned how to get her way with soft soap. This colloquial expression alludes to liquid soap, likening its slippery quality to insincere flattery. Its figurative use was first recorded in 1830.
American Heritage Idioms
2
general::
Phrase(s): soft soap
1. flattering talk; sweet talk. • I don’t mind a little soft soap. It won’t affect what I decide, though. • Don’t waste my time with soft soap. I know you don’t mean it. 2. (Usually soft-soap.) to attempt to convince someone (of something) by gentle persuasion. • We couldn’t softsoap her into it. • Don’t try to soft-soap her. She’s an old battle-ax.
McGrawhill's American Idioms And Phrasal Verbs