Adjective
not given to open expression of emotion
Source: WordNetThe first half of the 1960s was the apogee of what might be termed the Age of Cool - as defined by that quality of being simultaneously with-it and disengaged, in control but nonchalant, knowing but ironically self-aware, and above all inscrutably undemonstrative. Martin Filler
True bravery is quiet, undemonstrative. Philip Sidney