It's not "nudity", it's "nudity%3D1", which is just the escaped form for "nudity=1"
in "Advanced search" there's a tick box called "Adult filter", and "nudity=1" is only present if you don't tick that box
They may have chosen another word, but it's just a way of passing a search option. I don't know if there's deliberate search for a SEO effect.
Edit: @tabimura - I was typing the same thing as you were posting.