Singular and plural are identical denoting a substance, as with water.
Phonetically it is a corrupted form of "debris" perhaps combined with "dirty", i.e. debris + dirty = derby.
2 - She went back to work in the kitchen after using the bathroom without so much as washing the derby from her hands.
person two: O hey der by! just raking some seaweed, you?
person one: what in the hell did you just say?