While it might seem strange at first, "stern" in this context means "firm" or "rigid". The full breakdown is:
* "Con" - meaning "with" or "together"
* "stern" - meaning "firm" or "rigid"
* "ation" - a suffix denoting a state or action
So, "consternation" essentially means a state of being "firmly held together" or "rigidly fixed", often used to describe a state of dismay or anxiety that makes you feel stuck or immobile.