It's probably damage to the underlying nail bed. It might be, as suggested, a fungal infection as these can last for years. Or it may just be some bad cells that aren't replicating keratin properly. I had a slightly odd toenail that did something similar for many years. It eventually seemed to fix itself, but it took a very long time.

There are all sorts of conditions and diseases that show up as changes in the nails.

pca