In the story "Why I Live at the P.O," how is "cutting off your nose to spite your face" an ironic statement?
The scene you're referring to is another family disagreement at the dinner table. When Sister announces that she's had enough and is going to just live in the post office, everyone announces that they'll never set foot in the place again. Uncle Rondo accuses her of snooping through other people's mail and reading their postcards. Then Mama states that none of them will ever write another postcard. And That's when Sister says:
"Cutting off your nose to spite your face then," I says. "But if you're all determined to have no more to do with the U.S. mail, think of this: What will Stella-Rondo do now, if she wants to tell Mr. Whitaker to come after her?"
"Cutting off your nose to spite your face" is a figure of speech used to express a kind of self-destructive overreaction to a situation. If Stella-Rondo does keep her word and never write another postcard, then she'll have no way to contact her estranged husband. Remember, they didn't say they'd use some other post office; they said they'd never write another post card. Sister has won this argument because eventually they'll all have to back down on that vow.
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