A person whose face is usually expressionless (1885+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with poker face
A visage lacking any expression that can be interpreted, as in Whenever Betty attended one of her children's performances, she managed to keep a poker face. This term alludes to the facial expression of a poker player who is expert at concealing his feelings about his hand.
[ c. 1880