deadpan
Concise Oxford English Dictionary © 2008 Oxford University Press:
deadpan/ˈdedpan/
▶adjective impassive or expressionless. ▶adverb in a deadpan manner. ▶verb (dead-pans, dead-panning, dead-panned) say something amusing while affecting a serious manner.
'deadpan' also found in these Oxford entries:

