Green’s Dictionary of Slang

(orig. US) an expressionless stare.

1933		N. West Miss Lonelyhearts in Coll. Works (1975) 234: The dead pan broke and pain actually crept into his voice.
1943		P. Cheyney You Can Always Duck (1959) 53: This bar-tender is an interestin’ sorta guy. A dead pan. Nothin’ seems to worry him.
c.1950		K. Howard Small Time Crooks 49: Micky’s dead pan looked deader than usual.
1958		A. Sillitoe Sat. Night and Sun. Morning 169: This place’s like a graveyard. It’s full o’ deadpans.
1990		S. Morgan Homeboy 169: Hoe was [...] directing his best deadpan at the camera.
1999		Indep. on Sun. Culture 11 July 6: As a narrative, the picture is morally deadpan.