User blog comment:HanataSanchou/The Wano Country/@comment-53507-20150417170742/@comment-24789339-20150418070410

"... Nami is waving her hands, praying for the dragon to "fall for it". Brownbeard asks what she is doing, and Nami tells him she is doing a samurai impression, but Usopp corrects her saying she means ninja". - From Chapter 682 Long Summary.

That's the only mention of a ninja that I can remember.