yegg (n.)

also yegg-man, 1901, a word popular in the first decade of the 20th century and meaning vaguely "hobo burglar; safe-breaker; criminal beggar."

The great majority [of the Chicago criminal population] are what certain detectives call "Yegg-men," which is a term, by the way, that the detectives would do well to define. As far as I can discover it means tramp-thieves, but the average tramp seldom uses the word. Hoboes that break safes in country post-offices come under the Yegg-men classification. [McClure's Magazine, February 1901]

Popularized by the Pinkerton agency detectives. The 1900 "Proceedings of the 26th annual convention of the American Bankers' Association," whose members were protected by the Pinkerton's National Detective Agency, reported a letter dated Nov. 23 or 24, 1899, returning $540, taken earlier that year, to the Scandinavian-American Bank of St. Paul, Minn., noting that the thieves had been so hounded by detectives that they gave up the gains and advised the bank to advertise that it was a member of the American Bankers Association, because "the American Bankers Association is too tough for poor 'grafters.'" The letter supposedly was signed "John Yegg," but this was said to be a pseudonym and the report identified the man arrested later in the case as William Barrett.