cook up

Example: “The twins cooked up a plan to skip their piano lesson.”

Origin: "Cook" in the sense of fabricating or concocting something is attested in English from the 1600s, and the metaphor is well chosen for a specific reason: cooking is a transformation that happens out of sight of the person who will consume the result. Raw ingredients go in, something quite different comes out, and the eater has no access to what was done in between — which makes it an apt figure for anything assembled privately and presented as finished. The same logic produced "cook the books," also seventeenth-century, where accounts are worked over until they present an appearance unrelated to what went in. The phrase is not automatically dishonest, but it always implies a process the audience did not witness.