Marked by an impassive matter-of-fact manner, style, or expression.
1: a completely expressionless face.
2: a deadpan manner of behaviour or presentation.
To express in a deadpan manner.
“Cold enough,” Tristan hazarded, “to form a Bose-Einstein condensate?” “I love it when you talk dirty,” Oda said, so perfectly deadpan that I did a double take.” – The Rise and Fall of D.O.D.O., Neal Stephenson and Nicole Galland.