Where did I don't give a rats ass come from?

I Don't Give A Rat's Ass

It's just general slang that started up sometime during the 20th century. While nobody knows for sure how it came about, it's believed that a rat was used in the expression because of the way people view the rodent (annoying and small).

Source: Mindlesscrap

One theory: there used to be a bounty on rat's heads, and you could turn them in to the county office for a nickel a piece. This would make a rats bottom half worthless, thus "I don't give a rat's ass" would be appropriate.

The origin of " I don't give a rats ass" came from Pine City Mn. I used to say it all the time, a friend said his uncle liked the saying and started marketing products, I was in about 9th or 10th grade at the time year about 1973-74