poker faced
[adjective]
Synonyms: serious, earnest, grave, sedate, sober, sobersided, solemn, somber, staid, weighty, neutral, abstract, colorless, detached, disinterested, dispassionate, impersonal, unpassioned

[TahlilGaran] English Synonym Dictionary ▲

ˌpoker-ˈfaced adjective
showing no expression on your face:
a poker-faced officer
—poker face noun [singular]

[TahlilGaran] Dictionary of Contemporary English ▲