How to Say “What Exactly / On Earth” with 究竟 (jiūjìng)

究竟 (jiūjìng) is built from 究 (“to investigate”) and 竟 (“to the end”), so at heart it means investigating something to the very bottom. Its main job, and the reason you'll reach for it, is to sharpen a question into “what exactly?” or “what on earth?”: 这究竟是怎么回事？(“what exactly is going on here?”), 你究竟在哪里？(“where exactly are you?”). It's the same intensifying move as 到底, only a shade more formal and written. Where 到底 sounds like everyday speech, 究竟 fits an essay, a report, or a more careful, pointed tone.

The questioning 究竟 takes two shapes. The first is 究竟 + a question word (谁 / 什么 / 哪里 / 什么时候 / 为什么), a targeted question after a specific piece of information: 他究竟为什么迟到？(“why exactly was he late?”). The second is 究竟 + Verb + 不 / 没 + Verb, the choice question “…or not?”: 你究竟买不买？(“are you going to buy it or not?”), 他究竟有没有告诉你？(“did he tell you or not?”). Two rules ride along, both shared with 到底. First, a 究竟 question cannot take 吗: say 你究竟同意了没有？, not 你究竟同意了吗. Second, 究竟 belongs in the question, never the answer: you ask with it, you don't reply with it (for “I really don't know,” use 确实 or 真的).

Its second use is “after all.” Here 究竟 stops being a question and instead emphasizes a fact, in the fixed frame Noun + 究竟 + 是 + the same Noun， followed by the consequence: 老板究竟是老板，一句话就决定了一切 (“the boss is the boss, after all, one word from him settled everything”), 母语究竟是母语，说起来毫不费力 (“your mother tongue is your mother tongue, after all, it comes out effortlessly”). This works just like 毕竟 (bìjìng): the repeated noun says “that's simply what this is, and it explains the rest.” The repetition is the whole pattern; you can't swap in a bare adjective (孩子究竟很调皮 doesn't work, but 孩子究竟是孩子 does).

Because 究竟 and 到底 overlap so much, it's worth being clear on where they differ, which the comparison below lays out. Two gaps matter. First, only 到底 also means “finally” (公交车到底来了, “the bus finally came”); 究竟 never carries that sense, so don't use it for a long-awaited result. Second, only 究竟 doubles as a noun meaning “the actual truth”: 问出个究竟 or 弄个究竟 (“get to the bottom of it”), where 到底 can't stand. Everything else, the emphatic question and the “after all” 是-pattern, they share, with 究竟 simply the more formal choice.