"Gutter oil", digou you is any illegally recycled and refined oil. It's not necessarily from actual gutters: the "gutter" can be figurative. To be clear, does include, rarely and in the most egregious cases, literal sewer skimmings, but is also buying used cooking oil from food vendors (like from deep fat fryers and grill traps) and animal carcass rendering. Then refining and selling that oil for food use rather than as biofuel or other industrial uses. It can even apply simply to just using once-virgin frying oil for longer than regulations allow.