What does “到底” mean in Chinese questions?

到底 literally means “reach the bottom,” and in questions it drills for the real answer: 你到底要吃什么？ — “what do you actually want to eat?” In this clip a boyfriend offers Italian (too expensive), Thai (too sweet), and Chinese (she can cook that herself), gets 我都可以 (“anything works for me”) every time, and finally cracks: 哎呀，你到底要吃什么？ Add 到底 once the polite version of the question has already failed.

The “bottom” image runs through related words: 问到底 is to question something to the very bottom, and in statements 到底 means “in the end, after all” — 他到底还是来了, “in the end he came after all.” And note her answer: 我都可以, with 都 covering all the options — the sweetly infuriating “any of them is fine” that keeps the loop going.