What does it mean?

I'm reading a book, named "Darkly Dreaming Dexter" by Jeffry Lindsay. And I'm totally not following lost on one phrase.

First of all, a text from the book: Here's the context.

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I was adopted, raised by Harry and Doris Morgan, Deborah's parents. (...) Both dead now. And so Deb is the only person in the world who gives a rusty possum fart whether I live or die. For some reason that I can't fathom, she actually prefers me to be alive.
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And the phrase which I can't understand is "a rusty possum fart". What? What does it mean?

"possum" is short for an opossum.  A kind of a rat like animal.

"fart" is the air that is being expelled from your nether regions.

"rusty" - oxidation of metal by oxygen.  It also means useless.

So this sentence is a colorful way of saying, "I really don't care".  Question: Do you care if an opossum really fart?  "Rusty" is just another exaggerated description.  If you don't care about an opossum farting, then you really don't care if the opossum is also a useless one!