Chiefly a British idiom. When something "takes the biscuit" then it has become really bad, annoying or objectionable. Often used when something has worsened. Similar (but not identical) to the US version "that really takes the cake".
I know politicians are a bunch of lying fools, but that Donald Trump really takes the biscuit.
To be the epitome of something, typically something unpleasant. To stand as the best (or more likely worst) example of a series of things.
Of all the slimy schemes Jim's pulled over the years, his attempt to pay a psychologist to testify in court that Marie, whom Jim had actually repeatedly raped the previous year, was merely paranoid, just has to take the biscuit.