It means that you are
not interested in or
worried about something or someone; that you place no value on something, an event or a person; that you do not care and are
uninterested. It means that you place less value on it than you do on a turd, says it all really.
“
Malcolm had a
hissy fit when he found out he wasn’t invited to your birthday bash.”
“I
don’t give a shit.”