I don't give a--wait, what?

As I'm sure we've all heard the term, "I don't give a *insert what you don't give here*". I've used it, had it used on me, and know several people who use it when speaking to others. What perplexes me is the meaning of the phrase. Why would you give a fuck, shit,damn, rat's ass(huh?!), 2 shits or fucks(keep the shit),etc. to me as response or reaction to what I said to you? Picture that...Me:You know, I used to think you were cool; now I see you're not. Person:*hands me a rat's ass*. Me:*runs*. What?You wouldn't run if someone had ass of rat in their hand?Fuck keeping it G, I'm GONE! Any who, getting back to what I was saying, why on Earth would I want such a "gift"? Two shits?Not one, but TWO?! Gee, you're sooooo kind! Give me MORE shit please! I could really use as much shit as I can get; after all, my lawn is looking a little less green everyday! OK so maybe I don't want infinite amounts of shit(though I do think it'd make for greener grass); I'll definitely take you up on the two fucks though. I wonder, in the days of old, was there some nut job passing shit,fucks, and rat's asses to people who said or did things he/she didn't care about?My guess is yes. I'm a NYer so it wouldn't surprise me since there's people doing things like that today(gotta love the subway!).I will say this though, if you don't give a shit,damn,yada yada yada, I thank you. You have saved my hands from hours of sanitizing =).