Use the word deadpan to describe someone who uses no expression when speaking, such as the deadpan way some comedians deliver even their funniest jokes — which can make them even funnier

Pronunciation: /'dɛdpæn/
English Description: deliberately impassive in manner
Synonyms: expressionless
Chinese Translation: 面无表情的(mian4 wu2 biao2 qing2 de)
Spanish Translation: inexpresivo
ORIGIN: Deadpan dates to 1928, when pan was slang for "face." So if you seem to have a "dead face" as you say something, it means your face looks very blank — no energy or animation. Deadpan is associated with sarcasm, and like sarcasm, if you use it for comic effect, there's a risk your audience won't pick up on it. Nevertheless, deadpan humor can be funny and popular — witness the success of deadpan humor in the sitcom Seinfeld.

EXAMPLE SENTENCE:

My eyes aimed forward in a deadpan stare.
The young players treated the anarchic Fluxus works from the early '60s with an encouraging degree of genuine understanding and captivating deadpan humor.

*New word description, story and part of "EXAMPLE SENTENCE" are cited in Vocabulary.com