From China Digital Space
地沟油 (dì gōu yóu): ditch oil
Slop oil is “refined” from slop or swill, which is the leftover food that people and restaurants throw away. Usually, slop or swill is fed to pigs but some unscrupulous people will gather the slop and “recycle” the oil in the slop to sell as cooking oil. Refining slop oil is illegal because of the negative health effects. See here (Chinese) and here (English).