发臭 is a separable verb (离合词) meaning 'to stink' or 'to give off a bad smell'. It is formed by 发 ('to emit') and 臭 ('stink'). When you need to insert aspect markers like 了 or 过, or a duration, the two syllables can split: 发了臭 ('has started to stink'), 发过臭 ('has stunk before'). The object of the smell (e.g. 什么) can also go between them, though this is less common. In everyday use, the unsplit form 发臭 is most frequent.