cowardly: a shameful loss of nerve that is really a failure of faith, not the ordinary feeling of being afraid
This word never carries a good sense. It does not name the natural jolt of alarm that anyone feels facing real danger; it names giving in to that alarm in a way that is blameworthy. That is what sets it apart from the ordinary Greek word for fear (φόβος), which can be good, bad, or neutral depending on the situation, and from the reverent, careful fear of God (εὐλάβεια), which is a virtue. Among the fear-words this one is always an accusation.
What makes the fear blameworthy is that it is a failure of trust. When Jesus calms the storm, he calls the disciples cowards and in the same breath asks why they have so little faith. Their panic is condemned not because feeling afraid is wrong, but because it exposes what they lack in the presence of the one who commands the wind and the sea. The problem is not the emotion; it is what the emotion reveals.
The same charge sharpens in Revelation, where the cowards stand first in the list of those thrown into the lake of fire. These are not anxious or shy personalities. They are people who, when confessing Christ would have cost them their lives, chose safety instead. They are the opposite of the faithful believer who holds on and conquers.
In every use, then, the word names not a temperament but a decision: fear allowed to win out over faith. It is the kind of cowardice that sends a soldier fleeing from the battle line, borrowed and pressed into a spiritual meaning.