That grin is unsettling enough as it is with the way it literally never leaves the skeleton's face, but paired with that stare? It manages to make Wash feel at least minorly uncomfortable. At least, until Sans speaks again. Then his expression just sinks and his mood is straight back to agitated.

"Wow." Is all he has to say at first, squinting at the skeleton. But it seems too convenient, that joke. Terrible a joke as it is. "You planned that, didn't you."

Consider him unimpressed, unamused, and annoyed.