'Take the biscuit ' - an idiom?

[Duc]

Hello,
I've just read an exercise in my book that I find difficult. Here it is :
A number of common expressions include have or take + noun. Do you know what these mean?
1 Why don't you have a go?
2 Well, that really takes the biscuit!
3 I'll have to take the plunge and tell her.
4 She was always taking the mickey out of me
I don't know ''take the biscuit '' means.Can you help me,please :D
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Don't see your shade think you are great

[Mister Micawber]

.
1 Why don't you have a go? = make an attempt; take a turn
2 Well, that really takes the biscuit! = wins the prize; is the most interesting/shocking/admirable/best/worst example
3 I'll have to take the plunge and tell her. -- make the irrevocable decision
4 She was always taking the mickey out of me. -- teasing me; deflating my ego.
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Native English teacher at Mister Micawber's