straight face

a serious or impassive facial expression that conceals one's true feelings about something, especially a desire to laugh.

Origin of straight face

First recorded in 1890–95

straight-faced, adjective straight-fac·ed·ly

[streyt-fey-sid-lee, -feyst-lee] /ˈstreɪtˈfeɪ sɪd li, -ˈfeɪst li/, adverb

One that has been discovered very recently is that the straight-faced are the straight-laced.

How to Analyze People on Sight |Elsie Lincoln Benedict and Ralph Paine Benedict
The strength of long putts can generally be more accurately regulated with a lofted putter than with a straight-faced one.

The Soul of Golf |Percy Adolphus Vaile
"Mashy, a straight-faced niblich," is one of these definitions.

The Art of Entertaining |M. E. W. Sherwood
Haruhiku gave him a straight-faced glance that was a masked guffaw.

A Transmutation of Muddles |Horace Brown Fyfe

It is a plain, straight-faced iron with a round back, and is heavy, weighing sixteen ounces.

The Happy Golfer |Henry Leach

straight face

a serious facial expression, esp one that conceals the impulse to laugh

straight-faced, adjective

straight face

see keep a straight face.