In Death of a Salesman the phrase "Casting off" is used during a conversation where Linda and Willy are discussing their sons, who have come back to live with them.
WILLY: Figure it out. Work a lifetime to pay off a house. You
finally own it, and there’s nobody to live in it.
LINDA: Well, dear, life is a casting off. It’s always that way.
WILLY: No, no, some people- some people accomplish something.
Casting off is a knitting term also known as binding off. It is used to define an edge such as the bottom of a sweater or the top of a sock. The stitch is used to secure the last row that you have completed so you can concentrate on the next. Linda, being a homemaker, is more likely to be using this metaphor.
In Linda's mind, each trouble, joy, frustration, obstacle in life is like a row in knitting. Eventually the row ends and you cast it off and begin focusing on the next. Linda fixes the holes in her stockings, solving the problem- casting off. She finds the hose under the water heater and takes it- casting off. He yells at her, she supports him when her sons come to her rescue- casting off. She suspects he is having an affair, but he always comes home to her- casting off. She steadfastly made the payments to the house despite their financial hardships- casting off. Her husband loses his job- casting off. Finally, Willy commits suicide, her boys promise to continue what he started, and Linda has a final moment with her husband at his grave:
LINDA: Forgive me, dear. I can’t cry. I don’t know what it is, I
can’t cry. I don’t understand it. Why did you ever do that? Help
me Willy, I can’t cry. It seems to me that you’re just on another
trip. I keep expecting you. Willy, dear, I can’t cry. Why did you
do it? I search and search and I search, and I can’t understand
it, Willy. I made the last payment on the house today. Today,
dear. And there’ll be nobody home. (A sob rises in her throat.)
We’re free and clear. (Sobbing more fully, released.) We’re free.
(Biff comes slowly toward her.) We’re free... We’re free...
Rhetorically speaking, the device is a metaphor because the phrase states that one thing takes the place of another. In this case, life is not life, but a casting off.
What Linda meant by this is that life is not at all assured, nor predictable. It is the same meaning as if you threw your cards on a betting table: That is what "casting off" means.
In this case it is meaningful because it also refers to the act of casting off *as in "throwing your fishing pole for a fish to bite* and not knowing what will come out of the depths of the water.
Hence, Linda refers to the uncertainty of the future, but presents the excuse that life, EVERYONE's life goes through such uncertainty. Life is a casting off.