a bad egg

a bad egg

A dishonest or ill-behaved person. Primarily heard in US. We could tell he was a bad egg even at a young age.
See also: bad, egg
Farlex Dictionary of Idioms.

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.
See also: bad, egg
The American Heritage® Dictionary of Idioms by Christine Ammer.

bad egg

n. a repellent person. You’re not such a bad egg after all.
See also: bad, egg
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions
See also:
  • bad egg
  • bad trot
  • as bad as all that
  • have a bad opinion of (someone or something)
  • have a good, bad, high, low, etc. opinion of somebody/something
  • be downhill all the way
  • downhill all the way
  • it's the story of my life
  • story of my life, the
  • (it's/that's the) story of my life