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Are you talking about a large hex lock nut under the sink? A picture might help. You can post a pic here by clicking on ADD FILES next to the SEND button.
Assuming it isn't seized up you'll probably need a basin wrench (see attached pic) to get it off.
You don't have you grab the entire nut only part of the hex. There is no other tool available and I've never had a nut I couldn't get off with my basin wrench even the big one on old Moen faucets. But it's possible that your hardware store only has a cheap basin wrench which doesn't have a big enough jaw. If you can give me a picture I might be able you offer a work around or you can spend $35 for a good Ridgid basin wrench at Home Depot.
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