marked by or accomplished with a careful pretense of seriousness or calm detachment; impassive or expressionless: deadpan humor.
displaying no emotional or personal involvement: a deadpan style.
adverb
in a deadpan manner: He spoke his lines utterly deadpan.
verb (used with or without object),dead·panned,dead·pan·ning.
to behave or perform in a deadpan manner.
noun Also dead pan .
a face showing no expression.
a style of comedy that relies on the comedian's maintaining such a face.
Origin of deadpan
An Americanism dating back to 1925–30; dead + pan1 (in the slang sense “face”)
historical usage of deadpan
Deadpan is an Americanism from the 1920s. Around that time, pan was emerging as a slang term meaning “the face,” probably due to a similarity in shape to a frying pan. When your face is deadpan, it's expressionless, as if you were dead.