soft soap

1. A fluid or semifluid soap.
2. Informal. Flattery; cajolery.

noun

Excessive, ingratiating praise: adulation, blandishment, blarney, flattery, incense², oil, slaver. Idioms: honeyed words. See praise/blame.

verb - soft-soap

1. To persuade or try to persuade by gentle persistent urging or flattery: blandish, cajole, coax, honey, wheedle. Informal sweet-talk. See persuasion/dissuasion.
2. To compliment excessively and ingratiatingly: adulate, blandish, butter up, flatter, honey, slaver. Informal sweet-talk. See praise/blame.

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.

The noun has 2 meanings:

Meaning #1: flattery designed to gain favor
  Synonyms: blarney, coaxing, sweet talk

Meaning #2: a soft (or liquid) soap made from vegetable oils; used in certain skin diseases
  Synonym: green soap