不可不 is a fixed three-character expression meaning 'must' or 'have to'. It is formed by doubling the negative — 不 ('not') + 可 ('can be') + 不 ('not') — creating a double negative that results in strong necessity. Use it to emphasize an obligation or necessity in formal or written Chinese. Note that the pinyin of the headword is given as 'bú' but in natural speech the first 不 is pronounced bù and the second 不 before a 4th-tone syllable becomes bú (bù kě bú); the headword pinyin reflects the sandhi of the final syllable.