Useless, worthless, anything or anyone that has no purose, or works incorrectly... or fecal matter deposited out of the butt..
Moulton lava is not a piece of shit or a rock.
Elizabeth is a poisonious piece of shit.
There is a piece of shit on the side walk in San Fransisco.
Please clean that piece of shit before allowing it to go out in public.
Your car is a piece of shit.