Douglas Harper's Etymology Dictionary
1938, of persons, "with visage showing no emotion or reaction," from expression keep a straight face (1897), from straight (adj.).
Wiktionary
a. 1 Having a straight face, not revealing excessive emotion. 2 boldly, openly.
Usage examples of "straight-faced".
But for the stories by Broderick, Dedman, Love and a few others, the stories in Alien Shores share a sort of naive exuberance and unself-conscious straight-ahead storytelling that most of the Canadian pieces shunned -- the kind of straight-faced nonliterary audacity that characterizes the sf of the Golden Age.
We just happened to bump into each other," Bayley said, straight-faced.
Farid, straight-faced but with a gleeful, malicious sparkle in his eyes, said, "It's rumored that the production of daughters is down because certain things have trouble coming up.
She thanked him, straight-faced, and juggled the hot potatoes back to the chalet.
James was one of the few men to whom she knew she could say anything without fear of either shock, uncertain laughter, or—worse—that blankly incomprehending stare that young men gave her when she made some straight-faced joke.
Delighted news crews filmed the chaos while Ripperoons and assorted lunatics gave interviews to straight-faced camera operators about how Lord Jack was going to appear through an unstable gate created by a passing meteor and step off into the open Britannia amidst a host of unheavenly demons charged with guarding his most unsacred person from all earthly harm .