the mind > emotion > absence of emotion > [noun] > want of or incapacity for emotion > type or emblem (3)
As the type of what is lifeless, motionless, or void of sensation. Hence, a senseless or stupid person.
As a type of hardness, and hence as an emblem of insensibility, stupidity, deadness or the like; esp. in phrases of comparison with various…
Often used in similes as a type of inertia, senselessness, stupidity. Cf. sense 15: also post, n.1