Discard, cast, shed, molt, slough, scrap, junk mean to get rid of as of no further use, value, or service.
Discard literally denotes the getting rid of a card from one’s hand in a card game, usually because they are worthless or can be replaced by better cards; in its more common general sense, it implies a getting rid of something which one can no longer use to advantage or which has become a burden, an annoyance, or an interference.
Cast (see also THROW) may imply a seasonal process of discarding (as the throwing off of skin by a reptile). Especially when followed by off, away, or out, it more frequently implies a discarding, a rejection, a discharging, or a repudiation.
Shed is the ordinary, general term for the seasonal or periodic casting of skin, hair, antlers, or leaves.
The term is used also to imply a throwing off or discarding of anything that is a burden to carry, that represents a past stage in one’s development, or that is no longer useful or comfortable.
Molt is the specific term for the periodic shedding of feathers, skin, shells, hair, or horns by various animals and the growth of new corresponding parts. It often suggests a process of a change in plumage including the shedding of feathers and their renewal.
In general use molt even more often than shed implies change, flux, or transition.
Slough implies the shedding of tissue (as the skin by a reptile or, especially in intransitive use, of necrotic or cicatricial tissue from the surface of a sore or wound). The term is also common in the sense of to discard or throw off what has become objectionable, burdensome, or useless.
The last two words, scrap and junk, have literal reference to the throwing away of fragments, parts, or pieces that are useless to the owner or can no longer be used by him.
Scrap suggests a discarding as rubbish or refuse, but it may carry an implication of some use to another (as a processor or a dealer in parts or accessories).
Junk differs little from scrap except in stressing a throwing away and in carrying little implication of value to a second-hand dealer or to a processor of waste.