different between rattlebrain vs halfwit

English

rattle +? brain

rattlebrain (plural rattlebrains)

A rattlebrained person.
- 1924, Herman Melville, Billy Budd, London: Constable & Co., Chapter 13, [1]
  a genial young fellow enough to look at, and something of a rattlebrain, to all appearance.
- 1962, Isaac Bashevis Singer, The Slave, translated by Isaac Bachevis Singer and Cecil Hemley, New York: Farrar, Straus & Giroux, 1985, Part III, p. 297,
  My mother, peace be with her, when she called me a rattlebrain, was right.

From half and wit.

halfwit (plural halfwits)

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