deadpan

Short definition from Wiktionary

Adjective

1. 1
   Deliberately impassive or expressionless.
2. 2
   Having such a face or look.

Adverb

1. 1
   In a deadpan manner.

Noun

1. 1
   A style of comedic delivery in which something humorous is said or done while not exhibiting a change in emotion or facial expression.

Verb

1. 1
   To express (oneself) in an impassive or expressionless manner.