What a thing holds inside it. From the same root as contentment - to be content is to have one's wants bounded by what is already held.
Latin contentus, past participle of continere (to hold together, contain), from com- (together) + tenere (to hold); what a thing holds inside it, the same root that gives contentment as wants bounded by what is already held.
The kinship with contentment is not decoration: both descend from a holding-together, one applied to a container and one to a want. What a thing holds and what a person is satisfied with are, at the root, the same idea aimed two different ways.