I couldn't find your surname in the Oxford Dictionary of Surnames, but I'm pretty sure it means what it says: nettles. There are names like Nettleton or Nettlefield and they have to do with nettles.
There are also some place-names starting with Nettle- like Nettleworth, and they are all places where nettles once grew …
Not very appealing, but probably true.
Andy ;—)