From wordorigins.com

Benjamin Choate
Unregistered User
(5/30/02 3:50 am)
Yegg
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There were once the forms "John Yegg" and "yeggman". The words were associated with tramps (perhaps among other associations) ca. 1900. Claims about the origin include:

(1) from the name of a famous safecracker/criminal, John Yegg;
(2) from the German "Jaeger" = "hunter".

I do not know that either of these has any substantiating evidence.

The Pinkerton Web site gives another derivation (supposedly pre-1900):

**The word 'Yegg' or 'Yeggman' originated with the gypsies. When a particularly clever thief is found among a gypsy tribe, he is selected as the 'Yegg' or chief thief.**

I don't know that this is reliable at all. But note that many Romany dialects have something like "yek" meaning "[number] one" ... as do other Indic languages.