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.”
Not giving a Shit(Not caring)
I don’t Give a Shit! That You stole his Bike!