means The natural process of an organism losing an outer layer — hair, skin, leaves, or feathers — or, more loosely, getting rid of something gradually.

from From Old English 'sceadan,' meaning to divide, separate, or part — the same root that gives us 'shed' as in a watershed, where land parts the flow of water in two directions. The sense of an animal casting off its coat grew from this older idea of separation: to shed is to set something apart from yourself and let it fall away.