U.S. [Said to be the surname of a certain American burglar and safe-breaker.] A burglar or safe-breaker. So Yeggman.

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1900.  Inter Ocean, 13 May, 28/1. ‘In the past ten years,’ said Mr. Pinkerton, ‘there has sprung up in the United States a class of burglars known among their fellows as “John yeggmen,” but more frequently called “yeggs.”’

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1903.  N. Y. Even. Post, 23 June (Cent. Dict., Suppl.). The prompt breaking up of the organized gangs of professional beggars and yeggs.

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1905.  N. Y. Times, 2 Jan., 4/1 (ibid.). Detective Sergeants … captured on the Bowery three men who, they say, are among the most successful ‘yeggmen,’ or safe-crackers, in the business.

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1906.  A. Stringer, Wire Tappers, 100. ‘Now, nitro-glycerine I object to, it’s so abominably crude, and so disgustingly noisy.’
  ‘And so odiously criminal!’ she interpolated.
  ‘Precisely. We’re not exactly yeggmen yet.’

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