Another way of saying that you couldn't care a less. Similar to the expression i couldn't give a rat's ass, but you're targeting the muskrat's nuts which are its manhood and used in the production of musky perfume.
Nic: "I slept with a fifteen year old girl on the weekend and she had a tight virgin pussy!"
Jim: "I couldn't give a muskrat's left nut you wanker."
Jim: "I couldn't give a muskrat's left nut you wanker."
by Human Pickled Punk May 7, 2016