to melt away: to lose firm shape and dissolve, whether wax before fire, courage before terror, or a life dwindling to nothing
At its root the word describes a solid thing losing its firmness and running off like liquid. Manna melts in the morning sun, wax softens before a flame, and the mountains seem to flow like water when God appears. The picture never changes: what once stood firm can no longer hold its shape.
The Bible reaches for this image most often to describe courage draining out of people. When an army hears of an enemy it cannot beat, the people's "heart melts," the way wax loses its edge near a fire. This is the language of panic and collapse, not sorrow: it says that resolve has dissolved and the will to stand and fight is gone. It is the stock expression for the terror that falls on a nation facing judgment or invasion.
A third strand pushes the picture further. A person or thing can simply waste away, dwindling little by little until nothing is left. Here melting shades into perishing, the slow disappearance of something that once had real substance.
A close cousin, מוג, covers almost the same ground and the two are often paired; where this word pictures something dissolving, רפה pictures the hands going slack.