full: holding all it can, so that nothing more will fit and nothing needed is missing
At its core the word describes something as full: a basket packed to the brim, a body covered head to foot, a heart so taken up with something that it governs the person. The picture is always the same, of a thing filled to its limit, and it works just as well for what you can see as for what you cannot.
In the Gospels it usually points to a physical fullness, the baskets left over after the crowds are fed, or a man whose whole body is covered with disease. In Acts it turns almost entirely to the inner life. People are called full of the Holy Spirit, full of faith, full of good works, and also full of deceit or full of rage. In each case the word says that the quality has taken hold completely and now marks the person's character, not that it has topped them up for a moment. This is the state of being full, distinct from the action of filling or completing carried by the related verb πληρόω.
A smaller group of uses leans on the edge of the word that means whole or complete: the ripe grain that has finished forming in the ear, and a reward received in full rather than in part. Here full shades into finished, lacking nothing that would round it out.