"You know." He continues to wave his hand around. "Don't you?"

After an awkward second or two (or three!) he realizes what's happening here and he makes an 'ooh' sound. Wally has gotten so used to hanging out with the team, where there are fistbumps aplenty, that he may have forgotten that not everyone ever knows the majesty of the move. He puts his hands on his hips and grins at her.

"You really don't know how to fistbump? That's a shame."