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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.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
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.
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
n. a repellent person. You’re not such a bad egg after all.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.