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.
1 - He couldn't help at first being repulsed by the crack-whore as he handed her the crumpled $10 bill, even though he knew he would be submerged in her rancid derby soon enough.
2 - She went back to work in the kitchen after using the bathroom without so much as washing the derby from her hands.
