What it means
A bad apple is the dodgy one in a bunch, the person whose shady or toxic behaviour can foul the vibe for everyone else. You use it for the corrupt cop, the chaos-loving coworker, the student who keeps dragging the class into nonsense. It often pops up when people argue whether it's really just one bad apple or the whole setup's gone rotten.
Usage examples
"One bad apple on the committee soured the mood for everyone."
"They insisted it was just one bad apple, not a problem with the whole team."
"It only takes one bad apple in the team to turn every meeting into a sulk-fest."
"They blamed one bad apple, but the whole department had been cutting the same corners for years."
"Yeah, Dan's a proper bad apple. Leave him in charge for ten minutes and suddenly everyone's in trouble."
Where it comes from
It comes from the old English proverb that one rotten apple can spoil the barrel. That image had been around for centuries before bad apple got used for a corrupt or troublesome person. These days it often carries a sceptical edge, especially when people use it to pretend a wider problem is just one person's fault.
Other ways to say it
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