that (really) takes the biscuit idiom UK informal (US that takes the cake)
极其讨厌；十分惊人
You say that something or someone (really) takes the biscuit when it or they have done something that you find extremely annoying or surprising.
And you say she's opening your letters now? Oh, that really takes the biscuit! 你说她在拆你的信？啊，这可真令人难以置信！