居然 is an adverb meaning 'unexpectedly' or 'surprisingly', used to express mild shock or disbelief at something that contradicts expectations. Though the character 居 literally means 'to reside' and 然 means 'so' or 'thus', together they form a fixed expression with no literal connection to their individual meanings — it’s purely idiomatic. The word often introduces a clause or sentence that describes an outcome or fact that feels improbable, ironic, or contrary to prior assumptions.

It commonly appears in spoken and written Chinese, especially in informal or narrative contexts, and usually precedes the verb or predicate it modifies. Unlike stronger words like 竟然 (which implies deeper astonishment), 居然 carries a gentler, more conversational tone — often suggesting polite surprise rather than alarm. It frequently appears after conjunctions like 谁知道 or 没想到, but can also stand alone at the beginning of a sentence for emphasis.