- a person who is bad, dishonest, or unreliable; a good-for-nothing: a bad egg who had served several years in prison.
Origin of bad egg
An individual who turns out to be rotten, as in You can't trust him—he's simply a bad egg. Although egg had been used for various kinds of person (young, good, bad) since Shakespeare's day, this transfer of a seemingly wholesome food that, when opened, turns out to be rotten took place only in the mid-1800s. An early definition appeared in The Atheneum of 1864: “A bad egg ... a fellow who had not proved to be as good as his promise.” In contrast, the schoolyard saying Last one in is a rotten egg does not have any special significance other than as a way of urging others to join an activity, jump in the water, or the like. Also see good egg.