If you're lucky, it's just the nut and not the axle. If so, buy a new nut and consider using a serrated, or otherwise shaped washer under it for more bite on the frame end, so you won't have to over-tighten it to hold.
If the axle is also stripped, you may be able to salvage it by walking it across the hub a few millimeters to but a new section where the nut engages, otherwise it's a new axle too.
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