Impassive
Etymology
Formed from the prefix im- (not) and "passive," which traces to Latin "passivus" and the verb "pati" (to suffer, to feel, to undergo) — the same root as "passion." So the literal sense is "not feeling" or "incapable of being moved." English adopted it in the seventeenth century, and it settled on describing the outward absence of visible emotion.
adjective · /im-PASS-iv/
Showing no emotion or reaction; expressionless.
What it names
A face or manner that gives nothing away, whatever may or may not be felt underneath.
Example
The witness remained impassive as the verdict was read, her hands perfectly still.
Related idea
A stoic feels the blow and masters it; the impassive simply show nothing on the surface, whether or not anything stirs beneath.
See also