Lacking expression: Their faces remained expressionless as they listened to the bad news. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "expressionless" in a sentence
"It is characterized by an expressionless face, slow initiation of motor activity, loss of secondary movements (such as arm swing, which gives the patient a stiff, frozen appearance), and micrographia (handwriting becomes small and choppy)."
"Make your mouth straight not a smile, not a frown, just straight, what might used to have been called expressionless."
"It's just that ... you know how everyone still has an expression on their face even when they're so-called expressionless?"