This is an interesting question, I've looked around and there doesn't appear to be any significant reason behind the name. But you can read up about the history here:
I wonder if anyone else know the history behind the name,
I asked this same question when we had our original Nintex in office demo. The presenters had no idea either, and said the owners won't tell anyone where they came up with the name. I'm thinking it's some type of inside joke or slang term...which is why they probably don't want to tell anyone.