Definition - safecracker
There has been considerable speculation, none of it conclusive, regarding the origin of the word yegg. Theories include that it was the name (or a variant) of a notable safe-cracker, that it was borrowed from a Chinese dialect, and that it was simply a variation of some existing English word. We may not know exactly where the word comes from, but we do know that it came into use in the early 20th century, and has since fallen into a relative state of disuse.
Two yeggs blew the safe in H. C. Sass’ grocery and bakery at Rising Sun, O., and escaped with $400 in cash.
— The Democratic Banner, (Mount Vernon, OH), 1 Jan. 1915