Undemonstrative is a related term of impassive.
As adjectives the difference between undemonstrative and impassive
is that undemonstrative is not given to showing emotion or feelings; reserved or distant while impassive is having, or revealing, no emotion.
impassive
English
Adjective
(
en adjective)
Having, or revealing, no emotion.
Still or motionless.
Synonyms
* apathetic, phlegmatic