The superlative form of the adjective 'stolid', meaning someone who is not easily moved emotionally or mentally.
Being stolid is usually seen as a negative thing, like not caring about others or being insensitive.
Examples:
He is the stolidest person I know.
She is the stolidest of all the kids in the class.
The stolidest man in the world never smiles.
The stolidest cat in the world never plays.
The stolidest rock in the world never moves.
The stolidest tree in the world never changes.