Modern is going a bit too far – the word’s been used in reference to genitalia since the 18th century – but it’s definitely a post-medieval word.
To be honest, the word choice was less the problem for me than the tired Madonna/whore content of the line, the cringey flirtation between Nymeria Sand and Bronn, Nymeria’s really flimsy characterization, and the fact that the line didn’t really add anything to the scene.