Deadpan (adjective)

1. Having an expressionless face; impassive.
2. Not showing emotion or interest; deliberately unenthusiastic.

Origin:

This word is of uncertain origin, first recorded in 1920s american english, it may be derived from dead + pan, with reference to the expressionless face or to the idea of striking a flat surface.

Examples:

1. The comedian delivered the punchline with a deadpan expression.
2. He spoke in a deadpan voice, making it hard to tell if he was serious or joking.
3. She gave a deadpan response to the joke.
4. The newscaster broke the tragic news with a deadpan expression.
5. His deadpan delivery made the audience unsure if he was serious or not.