In Dulce et Decorum Est, hags is used to describe young soldiers. The simile 'coughing like hags' was used because the young soldiers went into battle strong and full of life. After battle, they returned looking aged and beat down - hence 'hags' or resembling beggars.
Answer:
Once in a blue moon.
Explanation:
This idiom mean very rarely. For example, a Hailey's comet comes once in a blue moon. It comes every 76 years. Which is very rarely.
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