Given their relative sizes, 9S's blow falls well short of Lambert's face and catches him in the side. He was half-expecting that, but he wasn't expecting the force behind it, so it gets a sharp 'oof' out of the witcher and makes him sit back up and away, no longer pinning 9S with a hand.

"Not bad," he tells him, instead of answering the question, because yeah Lambert's a real mature adult like that. A little winded, he pushes away from 9S and gets to his feet, using a hand to feel along his side.