This phrase is an idiomatic way of saying stop or quit usually when you are tired of doing something and don't want to do it anymore. For example, when you are trying to fix your car and you can't figure out how to do it, after a couple of hours, you can say "that's it. I'm calling it quits. I'm taking this to the mechanic tomorrow." In another example, if you are studying for an exam and it's very late, you can say "I'm calling it quits. I'm so tired. Let's continue tomorrow."
A: You know, I'm so tired of law school. I don't think I want to be a lawyer anymore.
B: Are you kidding me? You can't call it quits now. You're almost finished!
A: But what am I going to do with a law degree if I don't practice law?
B: Well, it'll look good on your resume. You have to finish school, then you can switch careers.
A: Easy for you to say. You've been thinking of going to med school since you were 5.
B: Actually, I've been thinking of calling it quits lately. It's tough on the family. I hardly see the kids anymore.
A: Wonderful! Let's call it quits on our careers together. Maybe we can open a restaurant, instead.
B: Very funny. ... Actually, that's not such a bad idea.
Alright. Whatever you do, don't call it quits if you're tired of English. Learn something new everyday, and practice as much as you can.
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