If you are deadpan, you look and sound completely serious while you are saying or doing something funny. Your face does not smile; your voice does not wink. The humour is that you give nothing away, and the other person has to catch up. Comedians use it a lot; friends use it when they tease without a grin. It can describe a person (a deadpan comic), a face or voice (deadpan delivery), or the way someone speaks (she said it deadpan). You will also hear a deadpan as the style itself, and a few people use it as a verb: he deadpanned the line. It is informal to neutral — everyday talk, reviews, comedy — not a stiff school word. Though actually, deadpan does not mean the joke failed, and it does not mean you are cold or unkind. It is a way of being funny that keeps a still face. Pan here is old slang for the face, so the picture is a face that looks “dead,” with no extra movement.