From China Digital Space
地沟油 (dì gōu yóu): ditch oil
Ditch oil, or “gutter oil,” is cooking oil that is “refined” from the leftover food that people and restaurants throw away. The leftovers are usually used to feed pigs, but some unscrupulous people will gather the slop and “recycle” the oil in the slop to sell as low-cost cooking oil. Refining slop oil is illegal because of the negative health effects; it is carcinogenic and contains hazardous chemicals.
The term “ditch oil” first became a topic of major concern in 2010, when authorities revealed that up to one-tenth of cooking oil used in China might actually be ditch oil. Ongoing investigations have shown that the ditch oil is widely used. The phrase “ditch oil” has since become a stand-in for China's ongoing food safety issues.