If one thing is a “deal breaker” it means that there is one thing that will make some decision final.
It means “If this one specific thing were true, there would be no deal”
If some guy really liked a girl’s personality but also thought she was too fat to be his girlfriend, he could say, “she’s really nice but I can’t date her. Her fat legs are a real deal breaker“.
That means, “Even if everything else was perfect about her, I could never date a girl with such fat legs. That alone is enough to stop me from dating her” (That sounds mean and unkind, but it’s a good example to explain the meaning of this term.)
Another example would be this:
“I love my job but I need to quit because the company is forcing my whole family to move to New York City if I want to keep the job. I hate big cities so that’s a deal breaker for me. I need to start looking for another job”.
So a “deal breaker” is one thing that is powerful or strong enough to change or stop a decision.
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