I have eaten
That were in
The poem begins with a confession. Clearly, Williams knows that the other person would not have liked him to eat the plums because they were carefully chilled in the ice box. Still, he goes ahead and does it anyway, despite the other person’s obvious intentions.
An icebox was the appliance people used before powered refrigeration devices. It is pretty much exactly what it sounds like: a big cabinet with ice in it. The ice had to be replenished by an “iceman,” who was kind of like a milkman, except for ice. The iceman cometh.
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