black stuff
Murky unnamed substance
Black stuff is an informal, often dismissive term for any dark-colored substance whose precise identity the speaker either doesn't know or doesn't care to specify: tar, oil, sludge, grime.
What's that black stuff on your hands?
In Irish and British slang, the black stuff specifically means Guinness stout, from its distinctive dark color.
stout
Heavy, dark brewed beverage
Irish · pub · tap · foamy · creamy · nitro · Dublin · settling · harp · widget
substance type (guinness context)
black stuffpale ale
Light-colored hoppy beer
lager · pilsner · light beer · white wine · water
black, stuff, blacks, blacker, blackest, blacken, blackened, blackening, blackness