the.com/deadpan

means Delivering humor or absurd statements with a completely flat, expressionless face and tone, giving no hint that a joke is happening.

from American slang from the early 1900s, bolting together 'dead' (lifeless, blank) and 'pan' — old slang for the face, likely because a pan is a flat, featureless surface, much like the look being described. The term caught on fast in theater and vaudeville circles for performers who delivered lines without a flicker of expression, letting the words do all the comic work.