Wash follows York in, comes close enough to hear what York is saying, and...he's treating the start of a conversation like it's the end of a conversation. Looks like someone didn't quite get the memo.

So Wash walks behind the couch, obviously heading to the snack table to finish up his interrupted work, and casually but purposefully whacks the back of York's head with the flat of his hand along the way. Shape up, asshole.